Question 340525
The fly take off toward the opposite cyclist, but at the same time the cyclist is coming toward the fly. 

This means we have 15t+10t=20.

25t=20

t=4/5 hrs or 48 minutes the fly has flown and the cyclist has pedaled.

In this time the fly flew 12 km and each cyclist traveled 8 km. 15(4/5)=12.

Since the cylists are pedaling 10 kph toward each other, in 1 hour they will meet. 

But as the fly heads back to the other cyclist, the cyclist is heading toward him at 10 km/hr.  

There is 4 km left to cover as the fly heads back to the cyclist it started out on.

In 12 minutes the fly covers 15(1/5)=3 km

Thus, the fly covers 12+3=15 km altogether.