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2018 preRMO p18, diopantine 4abc = (a+3)(b+3)(c+3), a+b+c =?

Source:

August 7, 2019
Diophantine equationdiophantinenumber theory

Problem Statement

If a,b,c4a, b, c \ge 4 are integers, not all equal, and 4abc=(a+3)(b+3)(c+3)4abc = (a+3)(b+3)(c+3) then what is the value of a+b+ca+b+c ?