The price of lithium increased by 80% in the first month of the year and by 240% in the first quarter of the year. By approximately what percent did it increase from February 1 to March 31?
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Wait, so 80% first month and 240% first quarter? I initially thought the 240% included the 80%, but no—the question asks from Feb 1 to Mar 31. Tricky wording. I got about 89%.
Yes, exactly. Q1 total is 240% increase, so the remaining increase is (1+2.4)/(1+0.8) - 1. That gives ~88.9%.
Ah, I see. So it's not 240-80. That's a common trap. Thanks!
Got 89% (D). Took me a minute to realize the 240% is for the whole quarter, not just the remaining two months. Good question.
Is the answer 89%? I did 3.4/1.8 = 1.888, so 88.8% increase. But some of my friends got 160% by just doing 240-80. Which is correct?
Yours is correct. Subtracting percentages only works if they're based on the same original value, but the 80% changes the base for the next increase.
What it tests
Your ability to compute percentage change, percentage of a whole, and to work backwards from a final value.
Common trap
Treating successive percentage changes as additive — a 10% rise then a 10% fall is not back to the start.