Think about how to set up the equation. In one hour one person is 6 miles from the start point and the other is 10 miles from it, so they are 16 miles apart. So they are becoming separated at a speed of 16 miles per hour. That speed is the sum of the speeds of each man.
Let d be their distance apart and h be the hours that has elapsed, then d=6h+10h or d=16h. If you want to know when they are 200 miles apart, then solve for d where 200=16d, or 200/16=d. That gives you d=12.5 or twelve and a half hours.
In that time one person travels 125 miles and the other travels 75 miles, a total of 200 miles.
