Use A for third graders, B for fourth graders, and C for fifth graders.
33 = A + B + C
Since there are 3 variables, you need to come up with two other equations to substitute in.
C = 6 + A Since there are six more fifth graders than third graders.
A = B Since there is an equal amount of third and fourth graders.
Since A is in both equations, then lets put each variable in terms of A.
B = A
C = 6 + A
So:
A + A + A + 6 = 33
A + A + A = 27 Since you subtract 6 from both sides.
3A = 27 Since you combine all three A's.
A = 9 Since you divide both sides by three.
Therefore, since A is a number of third graders, there are 9 third graders.
(Sorry if there's any typos.
