Let x and y be natural numbers, each less than 20, such that x, y, x + y and x – y are prime numbers. How many such combinations of (x, y, x + y, x – y) are possible?

Explanation

For x+y and x-y to be prime, one of them must be even (y=2). The source identifies two combinations: (5, 2) yielding primes 7 and 3, and (9, 2) yielding 11 and 7, pointing to two combinations.

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