When changing tires, the bicycle will travel x meters. If it is not changed, the front wheels will be scrapped after traveling 5000-x meters and the rear wheels will be scrapped after traveling 3000-x. At this time, when changing tires, the front wheels can only travel (5000-x) * (3000/5000) = 3 (5000-x)/5, and the rear wheels can travel again (3000-x). Therefore, there are:

3(5000-x)/5=5(3000-x)*3, x =1875m, that is, the bicycle went1875m when changing tires.

After the tire was changed, the bicycle left again: 3 (5000-1875)/5 = 5 (3000-1875) * 3 =1875 meters.

In this way, the bike went 1875+ 1875=3750 meters.

So the total * * * can exercise1875+1875 = 3750 (km).