The speedboat departs from Pier A and goes down the river. After passing Pier B, it continues to sail downstream to Pier C. After arriving at Pier C, it immediately drives back to Pier B in the opposite direction. It takes 10 hours. If A and B are 20 kilometers apart, The speed of the speedboat in still water is 40 km/h, and the speed of the river is 10 km/h, find the distance between B and C.
快艇从A码头出发，沿河顺流而下，途经B码头后继续顺流驶向C码头，到达C码头后立即反向驶回B码头，共用10小时，若A、B相距20千米，快艇在静水中航行的速度是40千米/时，河水的流速是10千米/时，求B、C间的距离．
<gadget id="calculator">20 * 10</gadget>
<output>200</output>

<gadget id="calculator">10 * 40 * 40</gadget>
<output>16_000</output>

<gadget id="calculator">10 * 10 * 40</gadget>
<output>4_000</output>

<gadget id="calculator">20 * 40</gadget>
<output>800</output>

<gadget id="calculator">10 * 40 * 10</gadget>
<output>4_000</output>

<gadget id="calculator">10 * 10 * 10</gadget>
<output>1_000</output>

<gadget id="calculator">4_000 - 800 - 4_000 - 1_000</gadget>
<output>-1_800</output>

<gadget id="calculator">200 + 16_000 + (-1_800)</gadget>
<output>14_400</output>

<gadget id="calculator">10 - 10</gadget>
<output>0</output>

<gadget id="calculator">40 + 40 + 0</gadget>
<output>80</output>

<gadget id="calculator">14_400 / 80</gadget>
<output>180</output>

<result>180</result>