A situation that describes opposite quantities that, when combined, make zero is when you have a certain positive amount and an equivalent negative amount. For example:
Imagine you have +5 apples and you give away 5 apples. The positive quantity of +5 (the apples you have) and the negative quantity of -5 (the apples given away) combine to give you:
+5 + (-5) = 0
In this case, the opposite quantities of +5 and -5 combine to make zero, illustrating how opposite quantities can balance each other out.