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How many of the prime numbers between 10 and 20 can be written as the sum of two primes?
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.
What is the smallest prime number that is the sum of two primes?
5
Is the sum of two prime numbers always a prime numer?
It is not. Of the infinitely many primes only one (the number 2) is even, the rest are all odd. The sum of any two primes other than 2 is even and therefore not a prime. If one of the primes in the sum is 2 then the sum is a prime only if the other is the lower of a pair of twin primes. So, while it is possible, it is certainly more likely that the sum is a composite.
What number is neither prime or the sum of two distinct primes?
One.
What two prime numbers equal 209?
There are no two primes whose sum is 209.
