2011 AMC 8 Problem 13

Attempt Problem 13 of the 2011 AMC 8 below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2011 AMC 8 solutions, or check the answer key.

All problems are used with official legal permission of the Mathematical Association of America (MAA).

13.

Two congruent squares, ABCDABCD and PQRS,PQRS, have side length 15.15. They overlap to form the 1515 by 2525 rectangle AQRDAQRD shown. What percent of the area of rectangle AQRDAQRD is shaded?
1515
1818
2020
2424
2525
Answer: C
Concepts:areapercentage
Small hint:
Compare the shaded rectangle area with the area of the 1515 by 2525 rectangle.
Big hint:
The overlap is a rectangle whose width is the sum of two side lengths minus 2525.
Solution:
We get that SC=DC+SR−DR=15+15−25=5.\begin{align*} SC &= DC + SR - DR \\ &= 15 + 15 - 25 \\ &= 5. \end{align*}
This means that the area of PBSCPBSC is 5⋅15=75.5 \cdot 15 = 75. The area of AQRDAQRD is 25⋅15=375.25 \cdot 15 = 375.
75375=15,\dfrac{75}{375} = \dfrac{1}{5}, which is 20%.20 \%.
Thus, C is the correct answer.
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