Two trains left the cities a and b at the same time and headed towards each other. the cities are s miles apart. the first train was traveling at the speed of v1 mph. the second train was traveling at the speed of v2 mph. after t hours the two trains met each other. find the formula for t in terms of s, v1 and v2. then find the value of t if: s=310, v1 = 75, v2 = 80