On a 40 miles * 10 mile rectangle, A move left 5 miles per hour, B move right 7 miles per hour, they all start at center of the top side of the rectangle, where their distance to the upper left corner would be 20 miles. They have a phone on their hands, which would only work in range of 30 miles. How long would they no longer hear from each other? After 6 hours, could they hear each other?