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.
x=-15 i think this is right i may be wrong though
