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