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jmc

number theory senior

Problem

Two circles, one of radius 5 inches, the other of radius 2 inches, are tangent at point P. Two bugs start crawling at the same time from point P, one crawling along the larger circle at inches per minute, the other crawling along the smaller circle at inches per minute. How many minutes is it before their next meeting at point P?
Solution
The circumference of the larger circle, , is . The circumference of the smaller circle, , is . The bug on crawls the circumference in minutes, while the bug on crawls the circumference in minutes. The two bugs will meet at point P in some minutes, when and are both integers. We have , so we have to find the LCM of and . The LCM is , so the bugs will next meet in minutes.
Final answer
40