The total length of the waterway between the two docks A and B is 80 kilometers. It takes Ship A 4 hours to travel downstream and 10 hours to travel upstream. If it takes 20 hours for boat B to travel upstream, what is the speed of boat B in still water?
A、B两个码头间的水路全长80千米，甲船顺流而下需要4小时，逆流而上需要10小时．如果乙船逆流而上需要20小时，那么乙船在静水中的速度是多少？
<gadget id="calculator">80 / 20</gadget>
<output>4</output>

<gadget id="calculator">80 / 4</gadget>
<output>20</output>

<gadget id="calculator">80 / 10</gadget>
<output>8</output>

<gadget id="calculator">20 - 8</gadget>
<output>12</output>

<gadget id="calculator">12 / 2</gadget>
<output>6</output>

<gadget id="calculator">4 + 6</gadget>
<output>10</output>

<result>10</result>