Last year a certain farm's cost for fertilizer was 15 percent of its

Question

Last year a certain farm's cost for fertilizer was 15 percent of its total costs, and its total costs were $200 per acre. If this year the farm's cost for fertilizer increases by $20 per acre and all of its other costs remain the same, approximately what percent of the farm's total costs will be its cost for fertilizer?

A. 19%
B. 21%
C. 23%
D. 25%
E. 27%

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Bunuel · Accepted Answer

Last year the cost for fertilizer was 15%*$200 = $30 per acre, and the other costs amounted to $200 - $30 = $170. This year, the cost for fertilizer increased to $30 + $20 = $50, while other costs remained at $170. Therefore, the percentage of fertilizer cost to the total costs is:

\frac{50}{50 + 170}*100 = \frac{50}{220}*100 = \frac{250}{11} \approx 23%  .

Answer: C.

GMATCoachBen · Answer

A lot of people get trapped with D, 25%, because they didn't account for the TOTAL costs changing as well. The question specifies that "all other costs remain the same", so the total costs must increase. Therefore, we will add the 20 to the denominator as well as the numerator:

.15 * 200 = 30

\frac{(30 + 20) }{ (200 + 20)} ≈ 0.23

To dodge these traps, a key habit is to read carefully and make sure to account for whether more than one variable is changing.

Another example of this is when something is removed. It could ask "There are 10 females out of 20 people, and 2 leave. What fraction of the REMAINING people are females?"

We'd need to make sure to subtract 2 from both the numerator and denominator:

(10 - 2) / (20 - 2) = 8 / 18

Or, in age problems: "In 10 years, Joe will be twice as old as Bill".  We need to add 10 to both the original Joe and Bill values:

J + 10 = 2 * (B + 10)

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Last year a certain farm's cost for fertilizer was 15 percent of its

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