Concept: The question tests on the basics of Relative Speed(Rates)
When 2 objects are in motion simultaneously, we use relative speed.
When objects are moving in the same direction, we subtract speeds.
Time = Distance / Relative Speed
relative Speed = (75 - 15) * 5/18 = 300/18 = 100/6 m/s
Time = 200 / (100 / 6) = 12 seconds