A ship travels 20 kilometers per hour at a speed of 3 kilometers per hour. How many hours will it take for the ship to sail 207 kilometers downstream and then 102 kilometers upstream?
一只轮船每小时航行20千米，水速为每小时3千米．这只轮船顺水航行207千米后再逆水航行102千米，共需多少小时．
<gadget id="calculator">20 + 3</gadget>
<output>23</output>

<gadget id="calculator">207 / 23</gadget>
<output>9</output>

<gadget id="calculator">20 - 3</gadget>
<output>17</output>

<gadget id="calculator">102 / 17</gadget>
<output>6</output>

<gadget id="calculator">9 + 6</gadget>
<output>15</output>

<result>15</result>