Question

How many numbers in the set {3, 13, 23, 33, . . .} can be written as the difference of two primes?

Answer 1

1

Explanation:

Notice that when we subtract two integers, the difference can only be odd if one integer is even and one integer is odd (even - even = even and odd - odd = even). If one integer is even, then that integer is divisible by 2 and thus not prime. The only exception is 2, the only even prime number. So one of the primes must be 2. If we add 2 to each number in the set to find the other prime, we end up with $\{5, 15, 25, 35, \ldots\}$. All of the numbers in the set are divisible by 5, which means the only prime number in the set is 5. So the only number in the set $\{3,13,23,33, \ldots\}$ that can be written as the difference of two primes is $5-2=3$. The answer is $\boxed{1}$ number.

Yes I copied and pasted the proper answer from a different site. I didn't want to write it out.

Answer 2

1

Explanation:

An odd number will be the difference of an even and an odd number. The only even prime is 2, so the other prime must end in 5. There is only one such.

Only 3 = 5 -2 can be written as the difference of primes.