52) 115-3=112 that were given away
Now divide 112 by each, seeing if they give you a non mixed number.
The only one that would give you a full number would be 28. So there are 28 kids in his class.
54) 19-4=15
Now double 15 since he sold half, so he started with 30 stamps originally.
56) 500-2*=324
-2*=-176
age=88