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2004 AMC 12A Problems/Problem 9

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The following problem is from both the 2004 AMC 12A #9 and 2004 AMC 10A #11, so both problems redirect to this page.

Problem

A company sells peanut butter in cylindrical jars. Marketing research suggests that using wider jars will increase sales. If the diameter of the jars is increased by 25\% without altering the volume, by what percent must the height be decreased?

\mathrm{(A) \ } 10 \qquad \mathrm{(B) \ } 25 \qquad \mathrm{(C) \ } 36 \qquad \mathrm{(D) \ } 50 \qquad \mathrm{(E) \ } 60

Solution

When the diameter is increased by 25\%, it is increased by \frac54, so the area of the base is increased by \left(\frac54\right)^2=\frac{25}{16}.

To keep the volume the same, the height must be \frac{1}{\frac{25}{16}}=\frac{16}{25} of the original height, which is a 36\% reduction \Rightarrow\mathrm{(C)}.

See also

2004 AMC 12A (ProblemsResources)
Preceded by
Problem 8
Followed by
Problem 10
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
2004 AMC 10A (ProblemsResources)
Preceded by
Problem 10
Followed by
Problem 12
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
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