10 years ago, age of mother was three times the age of her son. Say, the age of son was x and mother's age was 3x.
At present: Mother's age is (3x + 10) and son’s age is (x + 10)
After ten years: Mother's age will be (3x + 10) +10 and son’s age will be (x + 10) + 10.
Given that, mother’s age is twice that of son after ten years.
(3x + 10) +10 = 2 [(x + 10) + 10]
(3x + 20) = 2[x + 20]
Solving the equation, we get x = 20
(3x + 10): (x + 10) = 70: 30 = 7: 3.