Two cars go from a to b at the same time, A travels 80 kilometers per hour, B travels 72 kilometers per hour, B travels first for 0.5 hours, and A starts again. How long does it take for A to catch up with B?
两车同时从a到b，甲每小时行80千米，乙每小时行72千米，乙先行0.5小时，甲再出发，甲行几小时追上乙？
<gadget id="calculator">80 - 72</gadget>
<output>8</output>

<gadget id="calculator">0.5 / 8</gadget>
<output>0.0625</output>

<gadget id="calculator">72 * 0.0625</gadget>
<output>4.5</output>

<result>4.5</result>