Given a waterway of 80 kilometers, it takes Ship A 4 hours to travel downstream and 10 hours to travel upstream. If it takes 5 hours for boat B to travel downstream, how many hours will it take for boat B to go upstream.
已知80千米水路，甲船顺流而下需要4小时，逆流而上需要10小时．如果乙船顺流而下需5小时，问乙船逆流而上需要多少小时．
<gadget id="calculator">80 / 5</gadget>
<output>16</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">16 - 6 - 6</gadget>
<output>4</output>

<result>20</result>