Two horses begin running on an oval course at the same time. One runs each lap in 9 minutes; the other takes 15 minutes to run each lap. How Many minutes after the start will the faster horse have a one lap lead?  Choose the most appropriate option.
A) 36
B) 12
C) 9
D) 45/2
E) 3
The rate of the faster horse is 1/9 lap/minute;   The rate of the slower horse is 1/15 lap/minute;   Their relative rate is 1/9-1/15=2/45 lap/minute;   The faster horse to gain one full lap will need time=distance/rate=1/(2/45)=45/2 minutes.   Answer: 45/2.
<result>D</result>
Two horses begin running on an oval course at the same time. One runs each lap in 9 minutes; the other takes 15 minutes to run each lap. How Many minutes after the start will the faster horse have a one lap lead?