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Case interview questions: market sizing, guesstimates & brainteasers

Team Prosple

We give you examples of curve-ball questions you'll find in consulting case interviews and lead you step-by-step to craft the perfect answer.

What is a case interview?

If you’re a graduate chasing a career in consulting, chances are you’ll come across a case interview. Case interviews allow you to demonstrate how you think - your ability to understand a problem, break it down into its requisite parts, analyse them and communicate a solution. Check out Case Interview 101 to find out more about what a case interview is and seven tips on what to do in a consulting case interview. Below, we tell you more about types of questions you might be asked, so you can walk in, ready and prepared.

Marketing sizing and guesstimates

In these cases, you will usually be asked to ‘guesstimate’ the size of a particular market. Like business cases, there isn’t necessarily a ‘right’ or ‘wrong’ answer. The reality is that the interviewer probably has no idea what the real answer to the market sizing question is – nor do they even really care. These cases are simply a way for the interviewer to test your logical thinking and see if you are able to communicate and justify your rationale out loud.

Example market sizing questions

Here are some examples that have been asked in real-life consulting interviews.

  • How many kilometres of train tracks are there in Australia?
  • How many Whopper burgers does Hungry Jacks sell every year?
  • How many dogs are there in Australia?
  • How many people fly in and out of Sydney airport every day?
  • How many cups of coffee does the coffee shop downstairs sell every day?
  • How many soft drinks are consumed every year?
  • How many ads are shown on TV in Australia every day?
  • How many smartphones are there in the world?
  • How many toothbrushes are used in Australia every year?
  • How many light bulbs are there in Australia?
  • How many tyres are sold in Australia every year?
  • How many pieces of paper would it take to completely encircle the earth at the equator?

What’s common across all of these questions is that they can really be about anything! While some of these questions might seem unrelated to consulting, it’s actually extremely important! Consider that consultants in their everyday work typically need to come up with a reasonable assumption about the size and attractiveness of a market in order to come to a recommendation.

For example, say your client is a soft drinks manufacturer and is trying to decide if they should enter the Chinese market. Well, the answer will largely depend if they think there is enough demand for their product. One way to answer this is to segment the Chinese population according to certain demographics such as age, gender, income and lifestyle. While you might be able to find some of this information through publicly available sources such as government data, you may need to make some assumptions about the size of the target market. And that’s where the ability to construct a logical argument comes in.

A variation on the guesstimate question is to estimate revenue for a particular company or potential client. For example, estimate the annual sales for Woolworths’ brick and mortar stores in Australia or estimate the annual sales of Netflix’s online streaming subscriptions in the US.

Market sizing questions are often paired with a business case. For example, an interviewer may ask you to perform a small market sizing and then ask you some follow up questions about the company’s strategy or operations.

How to tackle the market sizing/guesstimate questions

When thinking through a market sizing question, ask yourself what information would you need to know in order to answer it. For example, let’s say the question is about sizing the surfboard market in Australia.

Questions you might like to ask yourself are:

  • How many people in Australia surf?
  • How many boards does a typical surfer own?
  • How often does a surfer get a new board?
  • Are there other purchasers of surfboards aside from individuals? For example, surf schools or hotel chains?
  • Is there a market for used surfboards?  

By systematically working through each of these questions, you will begin to demonstrate your ability to think through a problem – as well as demonstrate if your logic stacks up. For example, have you assumed that everyone across Australia surfs or only those in areas with beaches and prime surfing spots?

One way to start practising these types of questions is to think about the world around you in your everyday. Visiting your local coffee shop? Do a rough back of the envelope to figure out how many coffees they must sell every day. Or if you’re on the bus, consider how you might calculate how many buses there are on the road at any one time in your city or across the whole of Australia. Or heading out and need an umbrella? Think about how many umbrellas must be sold every year. Soon, you’ll be calculating everything about anything – be warned, it might just start getting addictive!  

It’s not necessary that you do a market sizing question all in your head. While proving your mental arithmetic is a bonus, it’s perfectly ok to use a pen and paper (or in some instances, you might be doing this on the whiteboard). In fact, we encourage you to write down the key numbers as you go along so that you don’t lose track of any important information. Be prepared to show your work so that you keep the interviewer engaged with you and able to correct you as required.

Also, pro-tip, don’t over complicate things for yourself. Use round numbers wherever possible. Why try to find 28.5 per cent of something when you could just say you’re going to round up to 30 per cent instead? Try to make the maths easier for you wherever possible.

Finally, remember to stay cool, calm and collected like the clinical cat we know you are. If you’re fazed by a guesstimate – and let’s face it, they can be pretty overwhelming – take a deep breath and just concentrate on deconstructing the question one bit at a time.

If you need a moment to think and gather your thoughts, ask your interviewer. Like we said earlier, an interviewer may try to stump you or throw you off by putting a spanner in the works. Or perhaps they’ll just cut you off and move on. This is all perfectly ok. Maybe they just want to see how you deal with unexpected changes, or maybe you’ve demonstrated enough ability that they’re ready to move on. Either way, try to have a little fun with it all – as crazy as that might sound! It’s all part of the fun and games of the consulting interview. After all, weren’t you always dying to know how many ping pong balls fit in an A380?

In summary, remember in order to nail the market sizing case, you must first, come up with reasonable assumptions; second, approach the problem clearly and logically and finally, crunch the maths correctly.

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Brainteasers usually come in the form of puzzles, riddles or logic questions. Most of the time there won’t even have a set answer. Again – and we know we harp on and on about this – but it’s all about the way you think. For the brainteaser particularly, the interviewer is testing your creativity and ‘out of the box’ thinking.

The old ‘why are manhole covers round’ (has done many rounds, thanks Microsoft) is a classic example of a brainteaser. In this instance, you might consider if there’s a structural reason for this. Maybe if the manhole was square, it would harder to fit within a cover because you would have to rotate it exactly the right way. Also, a round manhole cover won’t fall into a hole when rotated the wrong way so it’s much safer. Plus, if you need to move the manhole cover, you only need to roll it.

As you can see, unlike the business case or market sizing/guesstimate question, it’s much harder to prepare for a brainteaser. The point is, no brainteaser is ever quite alike. The best you can do is to keep a calm head and just have fun with it.

Types of brainteasers

Ok, while all brainteasers are unique snowflakes, it is possible to group them into specific categories. Although not all brainteasers will strictly fit into one of the categories below, these groupings might help you think how best to answer the questions asked of you.

Patterns or trend questions

We are used to these types of questions. They’re the ones that are often asked in IQ tests.

You’re given a series of numbers of letters and asked to figure out what comes next or what is the relationship between them.

For example, is there anything interesting about the following sequence of numbers? 8, 11, 5, 4, 9, 1, 7, 6, 10, 3, 12, 2, 0  

While we may be trying to figure out the maths, the answer is a little more out of the box. All the numbers are in alphabetical order: Eight, Eleven, Five, Four, Nine, One, Seven, Six, Ten, Three, Twelve Two, Zero.

Again, don’t always take something at face value and probe a little deeper to find the answer (although having said that, number patterns are also common, so be open to all the possibilities!).

Illusive questions

Think: if you’re a magician, you’re trying to draw your audience’s attention to an insignificant detail. This is exactly what the illusive question is all about. Sneaky! The best way to approach this is to make note of everything you’re being told.

Here’s a classic example: You are driving a bus and tracking the number of passengers on the bus. At the first stop, the bus picks up 26 people. At the second stop, 15 of those people get off the bus, and 8 new passengers get on. At the third stop, 2 passengers get off and 11 new passengers come on. At the fourth stop, 3 passengers get off and 4 passengers get on. What is the colour of the bus driver’s eyes?

We get it. You’ve been counting all the passengers thinking, ‘Yep, I’ve got this’, and then nek minit, they throw you this curveball question. Huh? Remember, you were driving the bus. Put your fancy calculators aside and look in the mirror if need be.

Another good example is this: Mary’s father has five daughters: 1. Nana, 2. Nene, 3. Nini, 4. Nono. What is the name of the fifth daughter? Nope, we know where you might have gone and it’s not Nunu (but well done on knowing all your vowels). Like the example before, the answer is in the first sentence. It’s Mary! Ahh, the magician’s done it again.

The thing is, these types of questions are designed to make you think you know where the brainteaser is heading…but it ain’t what you were thinking that’s for sure. That’s not to say you shouldn’t be writing down all those numbers but be open to the unexpected. The key is to recognise the ‘illusion’ and identify those important details you may have overlooked.  

Explanation questions

Well, Sherlock, it’s your time to shine. These types of questions provide you with weird and wonderful facts and it’s your task to explain how it all fits together bit by bit. It’s what they call ‘lateral thinking’. It’s all about creativity and a little imagination in these questions so don’t be shy in letting it all loose.

For example: In a tiny cabin in the woods, two men lay dead. The cabin is not burned but the woods around it burned. How did the men die?

In this instance, we may think about what ‘cabin’ really means. What if it’s the cabin of a plane and that plane crashed? Sounds like a good answer to me.

Like many brainteasers, there isn’t necessarily a ‘right’ or ‘wrong’ answer to this…although some answers may be more believable than others.

So, go ahead and come up with a wild but valid tale that will impress your interviewer, you storyteller you.   

Logical questions

With logical questions, usually there is no trick, illusion or creativity to these questions and they’re much more straightforward – and often require a bit of maths.

For example: What is the ratio of the weight of an elephant to the weight of an ant?

The answer is approximately one billion. The average elephant weighs about 3,000 kilograms compared to an ant which weighs about 3 milligrams.

3,000 kilograms = 3,000,000 grams = 3,000,000,000 milligrams.

While you may not know the exact weight of an elephant relative to an ant, you can definitely guess. If you arrive within an order of magnitude, you’ve done well!

Another example is: There are 12 black socks and 12 white socks together in a drawer. It is dark and you can’t tell them apart. What’s the smallest number of socks you need to pull out without looking in order to get a matching pair?

The answer is three socks. Assume the first sock is black. The second sock could be black, which means huzzah a matching pair. If however, the second sock is white, then the third sock will be either black or white and match the first or second sock.

How to approach the brainteaser

So, there you have it – some of the main types of brainteasers. Like we said though, it’s hard to prepare for brainteasers. It does help to practice a few, particularly those that are more in the logical, pattern or trends categories.

Reading through how brainteasers are structured and the types of things they ask you to do can help you be more mentally prepared for the different types. Other than that, we suggest just having fun with it – as best as you can!