I know the answer to this but can not explain to my child easily.
"one block has 15 recycling containers. There are three more paper containers than plastic containers. How many paper, how many plastic containers? Answer is 6 and 9.

Let A = number of paper containers and B = number of plastic containers. You know:

A+B = 15, since the total number of conatiners is 15.
A = B+3, since the number of paper containers is 3 more than plastic containers.

You now have two equations in two unknowns. You can substitute the value for A from the second equation into the first:

(B+3) + B = 15

Now solve for B:

2B + 3 = 15
2B = 15-3 = 12
B = 12/2 = 6.

Put this value for B back into the first equation:

A+B=15, so A+6 = 15
A = 15-6 = 9.

Hope this helps.