Triple-Prime

If a a , b b , and c c are positive prime integers satisfying the equation a b c = 23 ( a + b + c ) , abc=23(a+b+c), find the average of all possible values of a + b + c a+b+c .


The answer is 37.

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1 solution

Leonel Castillo
Jun 9, 2018

a b c = 23 ( a + b + c ) a b c 0 m o d 23 abc = 23(a+b+c) \implies abc \equiv 0 \mod 23 . Because the variables are prime numbers, at least one of the numbers must be 23 23 . Because of the symmetry of the equation we may assume that a = 23 a = 23 which turns the equation into b c = 23 + b + c b c b c = 23 ( b 1 ) ( c 1 ) = 24 bc = 23 + b + c \implies bc - b - c = 23 \implies (b-1)(c-1) = 24 . So ( b 1 ) ( c 1 ) (b-1)(c-1) is one of the positive factorizations of 24 which are 1 × 24 , 2 × 12 , 3 × 8 , 4 × 6 1 \times 24, 2 \times 12, 3 \times 8, 4 \times 6 . For the variables to be primes, it must be that the numbers in this factorization are one less than a prime and this condition is only satisfied by 2 × 12 , 4 × 6 2 \times 12, 4 \times 6 which yield the solutions ( 23 , 3 , 13 ) , ( 23 , 5 , 7 ) (23,3,13),(23,5,7) and any permutation of those two.

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