Two ships, A and B, depart from two wharfs 900 km apart at the same time, and after 6 hours, the distance between the two ships is still 480 km. It is known that ship A travels an average of 40 km per hour, how many kilometers per hour is the average travel of ship B.
甲、乙两船同时从相距900km的两个码头相对开出，经过6小时两船还相距480km．已知甲船平均每小时行40km，求乙船平均每小时行多少千米．
<gadget id="calculator">900 - 480</gadget>
<output>420</output>

<gadget id="calculator">420 / 6</gadget>
<output>70</output>

<gadget id="calculator">70 - 40</gadget>
<output>30</output>

<result>30</result>