Question 447136
You could use D = rt on each person, but the faster way is to note the difference in the speeds.


Since D is going 18 km/h faster than C, then relative to C, D is moving at 18 km/h. They are originally 9 km apart, so


9 km = (18 km/h)t --> t = 1/2 hr = 30 minutes.


(Note that when we used relative speeds, we were allowed to assume that C is stationary and D is moving at 18 km/h).