After five years the sum of ages of father and his son will be 70. Then four years ago sum of their ages was
52
F = father's present age
S = son's present age
After five years father will be F + 5 yrs old.
After five years son will be S + 5 yrs old.
After five years, the sum of their years will be 70.
F + 5 + S + 5 = 70
F + S + 10 = 70
F + S = 70 - 10 = 60
Four years ago father was F - 4 yrs old.
Four years ago son was S - 4 yrs old.
Mark sum of their ages four years ago as Σ
Four years ago sum of their ages was:
Σ = F - 4 + S - 4
Σ = F + S - 8
You already know:
F + S = 60
so
Σ = F + S - 8
Σ = 60 - 8 = 52