two cars start at the same time but from different starting points,one at an average speed of 100 mph and the other at 120 mph.if it takes the second car 15 minutes to catch the first one,how long was the head start given from the second to the first?译文：两辆车同时开始,但从不同的起点,一个平均速度为每小时100英里,另以120英里每小时.如果需要第二辆车15分钟赶上第一个,头开始被多长时间从第二到第一?