Tom's and Tim's ages add up to 21 years. In 3 years, Tom will be twice Tim's age. How old are they now?

Let X = toms age

let y = Tim's age

X+y=21

(X+3) = 2 (y+3)

X+3=2y+6

X-2y=3

2 * (X+y=21) multiple the equation by 2

so now you have 2 equations

x-2y=3

2x+2y=3

add the equations (2y cancel out)

3x=9

solve for X and plug back into one of the original equations to get y