Question 889428
Distance(d) equals Rate(r) times Time(t) or d=rt; r=d/t and t=d/r
The two horsemen are closing at the rate of 2+3=5m/sec
So, time it takes the two horsemen to meet: t=d/r=400/5=80 sec

That means the fly is flying for 80 sec.  At 8m/sec, this means the fly flies a distance of (d=rt)and d=8*80=640 meters.

Hope this helps---ptaylor