[aidenscott2016]

The point of the question isn’t trivia or mathematics, it’s to demonstrate how you deal with ambiguity

At work you may be asked to spec out and implement a poorly-defined defined request.

They expected you to have asked clarifying questions rather than steaming ahead in the assumption that the Thames is 50m metres wide at all point.

The interviewer stating “actually it’s 100m” shows that they didn’t understand this point either though, and was probably repeating the answer they were told to give if the candidate asks this clarifying question