A = 2B/3
C = 5A/4 = 10B/12 = 5B/6
根据B与C相差7有
B = 7÷（1-5/6） = 42
因此A = 2*42/3 = 28 ,C = 5*42/6 = 35
ABC三数之和 = 28 + 42 + 35 = 105