How do I find two prime numbers whose sum is 30?

We are looking for a sum of numbers- this means that both numbers have to be less than 30. Start by writing down all prime numbers less than 30: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29. Do you see any two numbers whose sum is 30? First notice that the ones-digit has to be 0: this means the possible combinations of ones-digits from our list of primes are 3+7 or 1+9. Looking now at the 10s digit, this needs to be 3 - 1 (as we carry over the one from the sum of 3+7 or 1+9). We are left with the pair (11, 19) and (23, 7).