Two percent changes in a row multiply. They do not add. Up 20% then down 20% is times 1.2 then times 0.8, which is 0.96, a 4% loss, not a wash. One-step change is in percent change.
Multiply the factors
Convert each percent to a multiplier: up 10% is 1.10, down 15% is 0.85. Multiply them, then multiply the original amount. Do not add +10 and −15 and call it −5.
A single “percent of” item is in percent problems. Combined work is a different two-step setup, in combined work problems. The short list is in ACT Math formulas.
The bottom line
Chain the multipliers. Adding the percents is the trap.
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