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.

Posted by Tim Fouracre

Tim founded Clear Books in 2008. Like many small business owners he worked from home for 15 months to get his startup off the ground. Today Tim enjoys helping Clear Books, its customers and its growing team innovate and achieve. Tim did his GCE O Levels in Ghana.


  1. 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?

  2. 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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