First let us calculate the total time the fly was in air. We know that the distance is 180 miles and the speed of the fly is 30 mph, therefore the time it took is:
time = 180 miles / 30 mph
time = 6 hours
Therefore the total distance that the fly travelled is (given 15 mph * 2 cyclists):
distance = 6 hours * 30 mph
distance = 180 miles