In still water, the speeds of two ships, A and B, are respectively 20 kilometers and 16 kilometers per hour. The two ships depart from a certain port successively, and B departs 2 hours earlier than A. If the water speed is 4 km/h, how many hours will it take for ship A to catch up with ship B after leaving?
静水中，甲乙两船的时速分别是20千米和16千米，两船先后自某港顺水开出，乙比甲早出发2小时．若水速是4千米/小时，甲船开出后多少小时追上乙船．
<gadget id="calculator">16 * 2</gadget>
<output>32</output>

<gadget id="calculator">4 * 2</gadget>
<output>8</output>

<gadget id="calculator">32 + 8</gadget>
<output>40</output>

<gadget id="calculator">4 - 16 - 4</gadget>
<output>-16</output>

<gadget id="calculator">20 + (-16)</gadget>
<output>4</output>

<gadget id="calculator">40 / 4</gadget>
<output>10</output>

<result>10</result>