In 1741 Christian Goldbach conjectured that every even number could be written as a sum of two primes. [At the time 1 was considered a prime and so 2 = 1 + 1.] The conjecture has not been proven so there may be even numbers which cannot be so written. Also every number which is greater than 5 and is one fewer than a multiple of 6 cannot be so written.
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There could be many answers for this question. One of them is 30 which is a sum of 15 and 15, both non-prime.
65 and 77
21 and 32
Prime numbers have two factors, composite numbers have more than two.
Prime numbers have two factors; composite numbers have more than two.
If one of the two numbers is 2, the sum of two prime numbers may sometimes be a prime number. Examples of this are 2 + 11 = 13, 2 + 521 = 523, 2 + 281 = 283. There are an infinite number of similar examples where the sum of 2 plus a prime is another prime. However if neither number is 2, then the sum of two prime numbers is always an even number greater than 2 and therefore composite.