A and B are running a 100-meter race. It is known that the ratio of A's speed to B's speed is 4:5. It takes 15 seconds for A to finish running. How many seconds does it take for B to finish running?
甲、乙两人进行100米赛跑，已知甲的速度与乙的速度的比是4：5，甲跑完用了15秒，乙跑完需用多少秒？
<gadget id="calculator">15 * 4</gadget>
<output>60</output>

<gadget id="calculator">60 / 5</gadget>
<output>12</output>

<result>12</result>