2007 AMC 12B Problem 6

Attempt Problem 6 of the 2007 AMC 12B 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 2007 AMC 12B solutions, or check the answer key.

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

6.

Triangle ABCABC has side lengths AB=5,AB=5, BC=6,BC=6, and AC=7.AC=7. Two bugs start simultaneously from AA and crawl along the sides of the triangle in opposite directions at the same speed. They meet at point D.D. What is BD?BD?
11
22
33
44
55
Answer: D
Concepts:relative speedperimeter
Small hint:
Together the two bugs cover the whole perimeter, so each crawls half of it
Big hint:
One bug travels A→B→D,A\to B\to D, a distance of AB+BDAB+BD
Solution:
The perimeter is 5+6+7=18,5+6+7=18, so each bug crawls 99 before they meet.
The bug going A→B→CA\to B\to C reaches DD on side BC,BC, having traveled AB+BD=9.AB+BD=9. Since AB=5,AB=5, we get BD=4.BD=4.
Thus, the correct answer is D.
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