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Keiko walks around track; find her speed

Source: AMC 10 2011 b Problem 12

February 24, 2011
AMCAIME

Problem Statement

Keiko walks once around a track at exactly the same constant speed every day. The sides of the track are straight, and the ends are semicircles. The track has width 6 meters, and it takes her 36 seconds longer to walk around the outside edge of the track than around the inside edge. What is Keiko's speed in meters per second?
<spanclass=′latex−bold′>(A)</span> π3<spanclass=′latex−bold′>(B)</span> 2π3<spanclass=′latex−bold′>(C)</span> π<spanclass=′latex−bold′>(D)</span> 4π3<spanclass=′latex−bold′>(E)</span> 5π3 <span class='latex-bold'>(A)</span>\ \frac{\pi}{3} \qquad <span class='latex-bold'>(B)</span>\ \frac{2\pi}{3} \qquad <span class='latex-bold'>(C)</span>\ \pi \qquad <span class='latex-bold'>(D)</span>\ \frac{4\pi}{3} \qquad <span class='latex-bold'>(E)</span>\ \frac{5\pi}{3}
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