Two prime numbers add up to 85. What is their product?

The total 85 is odd, and odd + odd = even, so one of the two primes has to be even — and 2 is the only even prime. That forces the partner to be 85 − 2 = 83, which is prime (it survives 2, 3, 5 and 7, and $11^2=121$ is already past it). The product is $2\times 83=166$. The trap $174=2\times 87$ comes from mis-subtracting 85 − 2 as 87; $170=2\times 85$ multiplies 2 by the TOTAL instead of by the partner; and 85 simply repeats the sum in place of the product.