A boat sails 20 kilometers per hour in still water. In a river with a current velocity of 4 kilometers per hour, it takes 12.5 hours to go back and forth between A and B. Find the distance between A and B.
一只船在静水中每小时航行20千米，在水流速度为每小时4千米的江中，往返甲、乙两码头共用了12.5小时，求甲、乙两码头间距离．
<gadget id="calculator">12.5 * 20 * 20</gadget>
<output>5_000</output>

<gadget id="calculator">12.5 * 4 * 20</gadget>
<output>1_000</output>

<gadget id="calculator">12.5 * 20 * 4</gadget>
<output>1_000</output>

<gadget id="calculator">12.5 * 4 * 4</gadget>
<output>200</output>

<gadget id="calculator">1_000 - 1_000 - 200</gadget>
<output>-200</output>

<gadget id="calculator">5_000 + (-200)</gadget>
<output>4_800</output>

<gadget id="calculator">4 - 4</gadget>
<output>0</output>

<gadget id="calculator">20 + 20 + 0</gadget>
<output>40</output>

<gadget id="calculator">4_800 / 40</gadget>
<output>120</output>

<result>120</result>