Question 504603
Let 
Present age of Shane = s
Present age of his daughter = d

Given
Shane is twice as old as his daughter
s=2d ............(1)
In 42 years means we have add 42 in the present ages of both
s+42=d+42 
then
ratio of thir ages will be 4:3 
4:3=s+42:d+42
4/3=s+42/d+42
4*(d+42)=3*(s+42)
4d+168=3s+126
4d=3s+126-168
4d=3s+126-168
4d=3s-42 .............(2)
Put the value of s from (1) to (2)
4d=3(2d)-42
4d=6d-42
4d-6d=-42
-2d=-42
Divide by -2 both sides of above equation
-2d/-2=-42/-2
d=21
Put the value of d in (1)
s=2d
s=2(21)
s=42

Shane is 42 and her daughter is 21 years old