The trains are approaching each other at a rate of 60+70 or 130 mph. [That's how fast the original 65 mile distance between them is shrinking.] Since time = distance/rate, the trains will collide in 65/130 hours which reduces to 1/2 hour. During that half hour, the fly, traveling at 110 mph, will have flown half of 110 miles or 55 miles. Edwin