Explanation:

Let the age of son is X and age of father 7X

Age of son after 10 years = X+10

Age of father after 10 years = 7X + 10

As per question, after 10 years;

3(X+10) = 7X + 10

3X + 30 = 7X + 10

4X = 20

X = 5 years

∴Age of father now = 7 * 5 = 35 years