A ship travels 20 kilometers per hour when sailing along the current and 15 kilometers per hour when sailing against the current. It is known that it takes 21 hours for the ship to go back and forth between ports A and B in the channel. What is the distance in kilometers between A and B?
一条船顺水航行每小时行20千米，逆水航行每小时行15千米，已知这条船在该航道的甲、乙两港间往返一次要21小时．甲、乙两港间的距离是多少千米？
<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">(1/20) + (1/15)</gadget>
<output>7/60 = around 0.116667</output>

<gadget id="calculator">21 / (7/60)</gadget>
<output>180</output>

<result>180</result>