Brain Teasers & Mental Math
A stock rises 25%, then falls 20%. Where does it end up versus the start?
Model answer
Exactly flat — back to the starting price.
Start at 100. Up 25% → 125. Down 20% of 125 → 125 × 0.80 = 100.
Why: 1.25 × 0.80 = 1.00, because 0.80 is exactly the reciprocal of…
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