A ship sails from point A to point B at a speed of 15 kilometers per hour. After the boat arrived at point B, it immediately returned to point A against the current. The speed of the boat was 10 kilometers per hour, and it took 2 hours longer than going from point A to point B. How many hours did it take for the boat to travel between A and B?
某船从A地到B地顺水航行，船速是每小时15千米．这条船到达B地后立即逆水返回A地，船速是每小时10千米，结果比从A地至B地用了2小时．这条船往返A、B两地共用了多少小时．
<gadget id="calculator">10 * 2</gadget>
<output>20</output>

<gadget id="calculator">15 - 10</gadget>
<output>5</output>

<gadget id="calculator">20 / 5</gadget>
<output>4</output>

<gadget id="calculator">4 + 4 + 2</gadget>
<output>10</output>

<result>10</result>