It takes 7 hours for a ship to sail 420 kilometers along the current, and the speed of the current is 15 kilometers per hour. How many hours will it take to return against the current?
一船顺水航行420千米，要用7小时，水流速度是每小时15千米，若逆水返回要用多少小时．
<gadget id="calculator">420 / 7</gadget>
<output>60</output>

<gadget id="calculator">60 - 15</gadget>
<output>45</output>

<gadget id="calculator">45 - 15</gadget>
<output>30</output>

<gadget id="calculator">420 / 30</gadget>
<output>14</output>

<result>14</result>