If , , and are positive prime integers satisfying the equation find the average of all possible values of .
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a b c = 2 3 ( a + b + c ) ⟹ a b c ≡ 0 m o d 2 3 . Because the variables are prime numbers, at least one of the numbers must be 2 3 . Because of the symmetry of the equation we may assume that a = 2 3 which turns the equation into b c = 2 3 + b + c ⟹ b c − b − c = 2 3 ⟹ ( b − 1 ) ( c − 1 ) = 2 4 . So ( b − 1 ) ( c − 1 ) is one of the positive factorizations of 24 which are 1 × 2 4 , 2 × 1 2 , 3 × 8 , 4 × 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 × 1 2 , 4 × 6 which yield the solutions ( 2 3 , 3 , 1 3 ) , ( 2 3 , 5 , 7 ) and any permutation of those two.