A is prime, a + 16 is prime, and a + 20 is prime. What is a?

A = 3 is obviously the solution, because 3, 19 and 23 are prime numbers
It is shown that there is no solution greater than 3
When the prime number is greater than 3, it can only be 6N + 1 or 6n-1
If 6N + 1, a + 20 = 6N + 21 = 3 (2n + 7) is a composite number
If it is 6n-1, a + 16 = 6N + 15 = 3 (2n + 5) is a composite number
So there is no solution greater than 3

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