Let the number of marbles Tim has = X
Then Sue has twice as many, so she has 2X
Jim has 15.
All three of them when added together have 63:
x + 2x + 15 = 63
Combine like terms:
3x +15 = 63
Subtract 15 from both sides:
3x = 48
Divide both sides by 3:
X = 48 /3
x = 16
Tim has 16 marbles.
