Tim has some marbles.
sue has twice as many marbles as tim.
jim has 15 marbles.
tim, sue and jim have a total of 63 marbles.
how many marbles does tim have?

We can solve this by setting up an equation.  I am setting the amount of marbles that tim has as T.  Because Sue has twice as many marbles as Tim, she will have 2T marbles.  We do not need a variable for Jim, because he has 15 marbles.  Now, we can set up this equation:

T + 2T + 15 = 63

Now, we can solve for T.  First, we subtract 15 from both sides.

T + 2T = 48

Next, we combine all the T terms.

3T = 48

Finally, we divide both sides to find how many marbles Tim has.

T = 16

So, Tim has 16 marbles.

Where's the rest of the