3 and 17, as well as 7 and 13.
1 + 19
3 + 17
7 + 13
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3 and 17
3 + 17 = 20
They are: 2+7+11 = 20
All prime numbers greater than 2 are odd numbers. For an odd prime to be written as the sum of two primes, one of the primes must be 2 because two odd primes will produce an even sum. 11 cannot be written as the sum of two primes. 13 = 2 + 11. 17 cannot be written as the sum of two primes. 19 = 2 + 17.
Not always as for example 20+9 = 29 which is a prime number