Problem 12 · 2016 Math Kangaroo
Medium
Fractions, Decimals & Percents
percent-multiplier
Petra has 49 blue pearls and one red pearl. How many of the blue pearls does she have to take away so that 90% of the remaining pearls are blue?
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Answer: E — 40
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Hint 1 of 2
The single red pearl never leaves; it must become 10% of what remains.
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Hint 2 of 2
If 1 red pearl is 10% of the new total, the new total is 10, so 9 blue remain.
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Approach: the unchanged red pearl fixes the new total
- The red pearl stays; afterwards blue should be 90%, so red is 10% of the pearls left.
- 1 red pearl = 10% of the new total means the new total is 10, so 9 blue pearls remain.
- Petra started with 49 blue and ends with 9, so she removes 49 − 9 = 40.
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