A remarkable solution, given that the fly will be 40 miles behind the first bike (and never overtake him) when the two bikes collide.
The source below gives the initial distance between the bicycles as 20 miles, the speed of the bikes as 10 MPH and speed of the fly as 15 MPH. Other than that it was a great story!
It’s not a particularly hard series to sum, the difficulty is construction the series, figuring out which series to sum. In my form it’s sum (1/5)^n, n from one to infinity. Then you have to multiply by 60, 4 being a scale factor that falls out, and 15 being the speed of the fly.
OK, fine; 5mph relative to each bike.
I was going from memory having heard it once over 40 years ago, and typing without coffee while on a work deadline.
Sheesh.