Cars A and B drive opposite each other from two places 480 kilometers apart. A travels 60 kilometers per hour, and B travels 40 kilometers per hour. Now, if the two vehicles meet at the midpoint between the two places, B must go first. how many hours.
甲、乙两车从相距480千米的两地相对开出，甲每小时行60千米，乙每小时行40千米，现在要使两车在两地间的中点相遇，乙必须先行多少小时．
<gadget id="calculator">480 / 2</gadget>
<output>240</output>

<gadget id="calculator">240 / 40</gadget>
<output>6</output>

<gadget id="calculator">240 / 60</gadget>
<output>4</output>

<gadget id="calculator">6 - 4</gadget>
<output>2</output>

<result>2</result>