Two horses begin running on an oval course at the same time. One runs each lap in 9 minutes; the other takes 18 minutes to run each lap. How Many minutes after the start will the faster horse have a one lap lead?  Choices:  A) 36  B) 18  C) 9  D) 4  E) 3
The rate of the faster horse is 1/9 lap/minute;   The rate of the slower horse is 1/18 lap/minute;   Their relative rate is 1/9-1/18=1/18 lap/minute;   The faster horse to gain one full lap will need time=distance/rate=1/(1/18)= 
<gadget id="calculator">1/(1/18)</gadget>
<output>18</output>
18 minutes.   Answer: 18
<result>B</result>
Two horses begin running on an oval course at the same time. One runs each lap in 9 minutes; the other takes 18 minutes to run each lap. How Many minutes after the start will the faster horse have a one lap lead?