Now

Janet -> 15

Mother -> 15 + 22 = 37

After x year

Janet -> 15 + x

Mother -> 37 + x

Given: Mother is twice as old as Janet

37 + x = 2(15 + x)

37 + x = 30 + 2x

x = 37 – 30 = 7

Mother will be twice as old as Janet after 7 years