Peter, Paul and Mary, whose walking rates are 1 ft/sec, 3 ft/sec and 6 ft/sec, start walking together in the same direction around a circular track that is 300 feet in circumference. After how many minutes are Peter, Paul and Mary together again?

250 seconds because it takes Mary 50 seconds to get around the track (300feet/6ft/sec) and it takes Peter 300 seconds to get around the track (300ft/1ft/sec) so if you subtract the fastest time from the shortest time you get the amount of time that Mary had to wait for Peter to get there, so after 250 sec (300sec-50sec) Peter, Paul, and Mary are reunited.