Question

Express the following as the sum of two odd primes.
18

Answer 1

Hint: In order to solve this problem we need to take two odd prime numbers whose sum is 18. All the prime numbers are odd except 2. Prime numbers are those natural numbers which have no other factor than 1 and the number itself.

Complete step-by-step answer:
The odd prime numbers before 18 are 3, 5, 7, 11, 13, 17.
Let us take it from the starting odd prime number and see whose sum is 18.
First we take 3 then to get 18 we need to add 15 which is not prime.
Then we take 5 then to get 18 we need to add 13 which is also an odd prime number.
Then we take 7, to get 18 we need to add 11 which is also an odd prime number. All the numbers we have taken if we take 17 then we have to add 1 which is not a prime number.
So there are two answers to this problem
13 and 5, 11 and 7
So,
18=11+7.
18=13+5.

Note: In this problem we have to consider all the odd prime numbers before 18 and analyze what we could add to get the sum 18. If the other number is also odd and prime then those two numbers will be the answer to this question. Proceeding like this you will get the right answer.