There is a circular runway, A runs a circle in 40 seconds, B runs in the opposite direction to A, and meets A every 15 seconds. How many seconds does it take for B to complete a lap?
有一个圆形跑道，甲用40秒跑完一圈，乙跑的方向与甲相反，每15秒遇到甲一次．乙跑完一圈需要几秒？
<gadget id="calculator">15 / 40</gadget>
<output>3/8 = around 0.375</output>

<gadget id="calculator">1 - (3/8)</gadget>
<output>5/8 = around 0.625</output>

<gadget id="calculator">15 / (5/8)</gadget>
<output>24</output>

<result>24</result>