Question 598089
Let Mary's present age = X years
So her mother's present age = 7X

After 20 years,
Mary's age = X + 20
Mother's age = 7X + 20

According to question (A.T.Q)
after 20 years, Mary will be half the age of her mother, OR
her mother's age will be the double of Mary's age

i.e. 7X + 20 = 2(X + 20)
7X + 20 = 2X + 40
5X = 20

So, X, i.e. Mary's present age, = 20/5 = 4 years