Answer:
Adult: 5
Kids: 3
Explanation:
In order to find the answer to this problem, we had to think of products of 3.
I first though of 6*3. This would be 18. But we cannot evenly put the children's tickets in that.
Next I did 3*5. This is 15. I tried adding 2 multiple times to get to 21.
17,19,21! I knew that is must be 5 and 3.
I hope I explained this enough. If you still don't get it, ask me another question. :)