What is the charge of an aluminum atom that has 13 protons, 13 neutrons, and 10 electrons?

Charge is +3.
The balance of protons and electrons is what affects the charge. The more electrons you have, the more negative the charge will be (this is only if you don't have the same number of protons and electrons ). If you have the same number of protons and electrons, that means the charge is neutral
In this picture you have 3 more protons.
Protons= positive
This means you will have a +3

You would do 1500/4 and the answer is 375