x + y + z = 142
3 x = 5 y = 7 z = k
3 x = k => x = k / 3
5 y = k => y = k / 5
7 z = k => z = k / 7
k / 3 + k / 5 + k / 7 = 142
35 k / 105 + 21 k / 105 + 15 k / 105 = 142
71 k / 105 = 142 / 1
71 k = 105 × 142
71 k = 14910
k = 14910 / 71
k = 210
x = 210 / 3 = 70
y = 210 / 5 = 42
z = 210 / 7 = 30