Suppose we call the digits ABCD. We have   D = 2*C   B = D - 5   A = C + B Expressing these in terms of C, we have   B = 2*C - 5   A = C + 2*C - 5 = 3*C - 5 We know every digit must … Read more

Multiples of 7 less than 40 are   7, 14, 21, 28, 35 Only 35 has a sum-of-digits that is 8.