Problem 418

We note that $6=2 imes 3$ and $27=3 imes 9$ and $39=3 imes 13$ and $77=7 imes 11$, which means that each of $6,27,39$, and 77 can be written as the product of two integers, each greater than 1. Thus, 53 must be the integer that cannot be written in this way. We can check that 53 is indeed a prime number.