Cars A and B are both traveling from place A to place B. Car A travels 60 kilometers per hour, and car B travels 40 kilometers per hour. After B travels first for 3 hours, how many hours can A car catch up with B?
甲、乙两车都从甲地到乙地，甲车每小时行60千米，乙车每小时行40千米，乙车先行3小时后，甲车开始行多少小时能追上乙车？
<gadget id="calculator">60 - 40</gadget>
<output>20</output>

<gadget id="calculator">3 / 20</gadget>
<output>3/20 = around 0.15</output>

<gadget id="calculator">40 * (3/20)</gadget>
<output>6</output>

<result>6</result>