There are two cargo ships, A and B. Ship A is traveling against the current 30 kilometers ahead, and Ship B is chasing behind. The still water speeds of A and B are 36 km/h and 42 km/h respectively. The speed of the water is 4 km/h. How many kilometers did Ship A travel and was caught up by Ship B? (the catch-up problem in the sailing problem)
甲乙两艘货船，甲船在前30千米处逆水而行，乙船在后追赶．甲乙两船的静水速度分别是36千米/时和42千米/时．水流速度是4千米/时．求甲船行多少千米被乙船追上？(行船问题中的追及问题)
<gadget id="calculator">42 - 36</gadget>
<output>6</output>

<gadget id="calculator">30 / 6</gadget>
<output>5</output>

<result>5</result>