Two children are on an escalator running downwards. The child at the bottom walks up at a speed of 10 steps per minute. The child at the top walks down at the same time at a speed of 5 steps per minute. The escalator is 90 steps long. How many minutes will they meet?
两位小朋友在一部向下运行的自动扶梯上，下两端．在底端小朋友向上走，速度是每分钟10级，字顶端的小朋友同时向下行走，速度是每分钟5级，自动扶梯长90级，他们两人几分钟后相遇？
<gadget id="calculator">10 + 5</gadget>
<output>15</output>

<gadget id="calculator">90 / 15</gadget>
<output>6</output>

<result>6</result>