At first, the problem seems impossible to solve. But once you use some logic, the solution is actually rather simple. Rattle your brain — or phone a friend —before you look at the solution below the picture.
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There is only one grammatically correct answer in the options. I was attracted to this approach by the fact that all three sentences as spoken by the characters in the problem are given in the present tense. That sent me on a search for a linguistic solution, rather than a mathematical or primarily logical one.