Lets say that Williams current age is x, since it is currently unknown
From here we know that in 15 years (x+15) he will be 3 times his age 5 years prior (3x-5). Thus we can set these equations up as equal to one another
1) x + 15 = 3(x - 5)
2) x + 15 = 3x-15
3) 30= 2x
4) 15 = x
5) Now this age is his current age of 15 years, but they also want his age 15 years from now
The equation is x + 15 = 3(x - 5)
This should be correct now, sorry for the misconception. Good luck