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About 10 years ago by Jay Scott

Mental Arithmetic


​Myths abound of heart-stopping interview questions. Like McKinsey’s “why are manhole covers round?”; “How many hairs are there on a dog?” or “How many golf balls could you fit inside a jumbo jet?”

But, outside the pressure chamber of a high-level interview, such thought experiments can be fun.

Here’s one:

You’ve been to a fruit market and loaded up with 100kg of fruit. But 99% of the weight of fruit is water and some of it evaporates on the long journey home. By the time you arrive, the fruit is 98% water. What is the weight of fruit you bring home?

At the start, the weight of solid (non-water) matter in the fruit is 1% or 1kg. When you get home, the same 1kg now makes up 2% of the total weight, which must therefore be 50kg.

Jay Scott is a consultant on the Interim team at FreshMinds Talent

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