## Thinking Mathematically (6th Edition)

$53$
The prime numbers less than 100 are: $\begin{array}{ccccc} &2 &3 &5 &7 &11 \\&13 &17 &19 &23 &29 \\&31 &37 &41 &43 &47 \\&53 &59 &61 &67 &71 \\&73 &79 &83 &89 &97 \end{array}$ Notice that the consecutive prime numbers $47, 53, 59$ are the only three consecutive prime numbers that are equally spaced. Thus, the middle-aged man must be $53$ years old.