The product of two odd prime numbers is 51 What are they?

the clue is that one of the factors must be <= square root(N). So, you know one number must be <=7 and there are only 4 possible primes that are eligible. They are 2,3,5 and 7. Because 51 is odd, we toss out 2. The number 5 only evenly divides into numbers ending in 0 or 5, so we can toss out 5.

Then, the numbers are either 3*(51/3) or 7*(51/7). Only one of the expressions in parentheses is also a Prime number. This gives your answer for the pair.