The total distance of fast swimming is 5×60×0.8=240, and it takes less than five times to swim to the opposite side four times. On the basis of fast swimming, the distance between slow swimming and the opposite side increases by 12.5m (i.e. 50-0.6× 50/0.8). Meet for the fourth time, and the fastest one will reach the opposite side for the fourth time. (The slow one arrived for the third time) After that, both of them set off for the last leg at the same time (neither of them swam to the opposite side). The slow one stays behind until the end.

So the two met four times.