Cars A and B set off at the same time from cities A and B, which are 540 kilometers apart, and head toward each other. The distance traveled by car A per hour is 15 kilometers less than that of car B. It is known that the speed ratio of the two cars is 4:5. How long does it take for the two cars to meet?
甲、乙两车分别从相距540千米的A、B两城同时出发，相向而行．甲车每小时行的路程比乙车少15千米，已知两车的速度之比为4：5，求两车几小时相遇？
<gadget id="calculator">5 - 4</gadget>
<output>1</output>

<gadget id="calculator">15 / 1</gadget>
<output>15</output>

<gadget id="calculator">4 * 15</gadget>
<output>60</output>

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