Two boats, A and B, sail upstream from port A respectively. In still water, boat A travels 15 kilometers per hour, while boat B travels 12 kilometers per hour. Starting to set off, when ship A catches up with ship B, how many kilometers has it left port A?
甲乙两船分别从A港逆水而上，静水中甲船每小时行15千米，乙船每小时行12千米，水速为每小时3千米，乙船出发2小时后，甲船才开始出发，当甲船追上乙船时，已离开A港多少千米．
<gadget id="calculator">15 - 3</gadget>
<output>12</output>

<gadget id="calculator">12 - 3</gadget>
<output>9</output>

<gadget id="calculator">12 - 9</gadget>
<output>3</output>

<gadget id="calculator">9 / 3</gadget>
<output>3</output>

<gadget id="calculator">2 * 3 * 12</gadget>
<output>72</output>

<result>72</result>