Question

Keith is thirteen years older than nephew William. In four years Keith will be twice as old as William. How old is William now?

Answer 1

Let's call Keith x and William y
x=13+y

4+x=2y

substitute x with 13+y:
4+(13+y)=2y
17+y=2y
17=y

William is 17 years old

Answer 2

William is 24 years old. (I think)