My favourite interview question: ‘How many petrol stations are there in the UK?’

Let’s get the answer out of the way first because it’s not really the point of the piece - according to Google, as of December 2017, the UK has 8,422 petrol stations. 

Of course, Googling the answer would be far too easy an option for an interview…and, while I’m not denying that I’d be impressed by someone who knew this statistic off the top of their head, what I’m really looking for is 10-15 minutes of interesting discussion about things like Estimates (how many cars there are and the average size of petrol stations); the Maths (how these figures can be put together to get something close to the number of petrol stations) and the Constructive Challenge bit (i.e. what else they might try).  

Let’s consider each part in turn.

 ESTIMATES

Even being willing to make any guess at all is a challenge for some candidates, but I think it’s worth persisting with this line of questioning. Dealing with the ambiguity of working without complete information is a skill we all need in our jobs, as is being willing to try something new. Sometimes, candidates are willing to have a go but then really struggle to generalise, eg ‘but all petrol stations are different sizes’. I do as much as I can to encourage them to move past this but, if they really can’t, this is a sign to me that they’ll struggle to see the bigger picture or improvise solutions in their roles.

MATHS

While not a maths test, as such, the question gives an idea of how comfortable people are with numbers. Under the pressure of an interview, it’s entirely possible for a mathematical expert to accidentally calculate 60% of 60 million and come up with 360,000. But when I ask them to look at the answer they got, candidates who are more naturally mathematical will spot their mistake. 

CHALLENGE

The very best candidates reach the end of the calculation and, almost without being prompted, start suggesting how they could refine their estimates – what they might now do differently. They also tune into the suggestions and questions I’m asking throughout. If, at the end of the question, I feel that it’s been an uphill battle in getting the candidate to engage with me on the question, it suggests that, at some point in the job, I’d end up feeling the same way about them on a non-petrol-station-related topic.

 The number of petrol stations in the UK is certainly an obscure question! But, time and time again, it’s what gives me the most information about a candidate. And - yes, there are candidates out there who excel at all the above.  

What’s your favourite interview question?