I'll do the first three problems to get you started
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Problem 1
Start things off by taking 1 crate from each side. If you do the same thing to both sides, then things stay balanced.
After this happens, you'll have only one crate remaining on the left side, and there are 2 rocks on the left side as well. Move these 2 rocks over to the right side. Taking away from the left and adding the same amount to the right keeps things balanced.
By this point, we have a crate on the left side and 6 rocks on the right side. Therefore, each crate has 6 rocks in it.
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Problem 2
Move the 2 rocks on the left side over to the right side. The right side goes from 18 rocks to 20.
Divide the 20 rocks among the four crates, so 20/4 = 5 meaning there are 5 rocks per crate.
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Problem 3
Similar to problem 1, we will take crates away. This time exactly 2 crates will be taken from each side. You will be left with a crate on the right side all by itself. There are 12 rocks on the left side. So one crate has 12 rocks.