Two trains, A and B, leave opposite each other at the same time from two cities 540 kilometers apart. The speed ratio of the two cars A and B is 4:5, and the car A travels 60 kilometers per hour. After how many hours can the two cars meet?
甲、乙两列火车同时从相距540千米的两城相对开出．甲、乙两车的速度比是4：5，甲车每小时行60千米，经过几小时两车能相遇？
<gadget id="calculator">60 / 4</gadget>
<output>15</output>

<gadget id="calculator">15 * 5</gadget>
<output>75</output>

<gadget id="calculator">60 + 75</gadget>
<output>135</output>

<gadget id="calculator">540 / 135</gadget>
<output>4</output>

<result>4</result>