datonman wrote:Here's the question:
The output of a factory is increased by 10% to keep up with rising demand. To handle the holiday rush, this new output is increased by 20% by approximately what percent would the output of the factory now have to be decreased in order to restore the original output?
A) 20%
B) 24%
C) 30%
D) 32%
E) 70%
Let the original output = 100.
After a 10% increase, the new output = 100 + 10% of 100 = 110.
After a further 20% increase, the holiday output = 110 + 20% of 110 = 132.
By approximately what percent would the output of the factory now have to be decreased in order to restore the original output?
Many students get confused when answering this type of question.
One way to avoid making a silly mistake is to PLUG IN THE ANSWERS.
When the holiday output of 132 is decreased by the correct answer choice, the result will be an output of 100 (the original output).
Start with an easy value in the middle.
Answer choice C: 30%
Holiday output reduced by 30% = 132 - 30% of 132 ≈ 132-40 = 92.
Here, the decrease is too BIG.
Since a smaller decrease is required, eliminate C, D and E.
Answer choice A: 20%
Holiday output reduced by 20% = 132 - 20% of 132 ≈ 132-26 = 106.
Here, the decrease is too SMALL.
Eliminate A.
The correct answer is
B.
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