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">20 + 4</gadget>
<output>24</output>

<gadget id="calculator">20 * 12.5</gadget>
<output>250</output>

<gadget id="calculator">4 * 12.5</gadget>
<output>50</output>

<gadget id="calculator">250 - 50</gadget>
<output>200</output>

<gadget id="calculator">20 - 4</gadget>
<output>16</output>

<gadget id="calculator">20 + 4 + 16</gadget>
<output>40</output>

<gadget id="calculator">200 / 40</gadget>
<output>5</output>

<gadget id="calculator">24 * 5</gadget>
<output>120</output>

<result>120</result>