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12Mar 13

A curveball question to put a candidate on the spot


No calculator, pen or paper permitted. What is 49 squared?

This is the question I ask interviewees at Clear Books. I particularly like this question because someone asked me the same question in an interview when I was at KPMG so I can benchmark against my own response.

It’s great for putting the pressure on, seeing if the candidate can do some sums in their head and also if they can think logically to simplify the problem.

  • Jeremy McMinn

    Loved this Tim – my first thought was multiply 49 by 50 then subtract 49. If under pressure, my response may have been different.

    • Tim Fouracre

      Jeremy – you’ve got the job :)

      I’ve had a variety of correct and incorrect answers over the years.

      Common mistakes are mishearing the question and giving the square root – 7. Some people try and do 40^2 + 9^2.

      One person has actually done the straight multiplication in his head – amazing!

      Lot’s of people take an approach of 50^2 but then many don’t know how to work backwards to the answer.

      I’m going to need a new question now… any suggestions?

  • IanP

    Along the same lines but some of these are a little bit more technical, which I prefer as it allows you the chance to see the logic and assumptions they have used:

    i) How many people are born in the UK each day? (I like this one but there is only one really successful way to tackle it so if you don’t hit upon this then it can be a bit messy)

    ii) How many people use Heathrow each day?

    iii) How many petrol stations are there in the UK? (my personal favourite, it’s very difficult to get any “purchase” on the problem as there is so little to really grab hold of, you have to start with some reasonable broad assumptions).

    iv) If you have a cube of bricks, 10 by 10 by 10, and you remove the entire outer layer of bricks, what percentage of bricks remain? (This is probably most similar to your question as it’s clearly got a right answer so you can see how different people approach it – also quite a surprising answer!!).

    No calculator, although I would suggest a pen and paper for the first three might be quite useful.

    • Tim Fouracre

      Hi Ian,

      Yep I’ve seen the petrol one before – it’s a good one.

      Another similar one – what is the probability that two dogs in the UK will have the same number of hairs on their body?

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