A can run around a circular track in 40 seconds. B runs in the opposite direction and meets A every 15 seconds. How many seconds does it take for B to run a circle?
甲用40秒可绕一环形跑道跑一圈，乙反向跑，每隔15秒与甲相遇1次，乙跑一圈所用的时间是多少秒．
<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">1 / 40</gadget>
<output>1/40 = around 0.025</output>

<gadget id="calculator">(1/15) - (1/40)</gadget>
<output>1/24 = around 0.041667</output>

<gadget id="calculator">1 / (1/24)</gadget>
<output>24</output>

<result>24</result>