Question 261437
Let the age of Sally = x
Then the age of Joe  = x+5
Also, in 15 years Joe will be 3 times as old as Sally is now.
    x+5+15 = 3x
    x+20 = 3x
    20 = 3x-x
    20 = 2x
    2x = 20
    x = 20/2
    x = 10 years