father = f
son = s
The present age of son is half the present age of his father
s = 0,5 x f
10 years ago the father was thrice as old as his son.
f-10 = 3(s-10)
i replace s by "0,5xf"
f - 10 = 3 (0,5f - 10)
f - 10 = 1,5f - 30
0,5f = 20
f = 40
=> father = 40 and son = 20
10 years ago
f = 30 et s = 10 (f = 3x10) ok