Posted by **Tom** on Tuesday, June 11, 2013 at 5:46am.

Alvin, Calvin and Melvin are running laps around a track. Alvin runs a lap every 4 minutes, Calvin runs a lap every 6 minutes and Melvin runs a lap every 9 minutes. If they start off at the same point, how many minutes will it take before all three of them are back at the starting point?