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Try a numerical example. If the base price of 80 goes up by 20%, what's the new price? If then the price goes back down to 80 from the new price, what is the percentage change?

If that does not give you a general formula, try some other numeric examples. If nothing clicks, show us your examples.

Experimentation is frequently how formulas are discovered although, in this case, it can also be discovered by pure algebra.