The distance between A and B is 680 kilometers. The ordinary bus from A to B travels 60 kilometers per hour. After 2 hours, the express train travels 80 kilometers per hour from B to A. The express train leaves How many hours later will the two cars meet?
甲、乙两城相距680千米，从甲城开往乙城的普通客车每小时行驶60千米，2小时后，快车从乙城开往甲城，每小时行80千米，快车开出几小时后两车相遇？
<gadget id="calculator">60 * 2</gadget>
<output>120</output>

<gadget id="calculator">680 - 120</gadget>
<output>560</output>

<gadget id="calculator">60 + 80</gadget>
<output>140</output>

<gadget id="calculator">560 / 140</gadget>
<output>4</output>

<result>4</result>