It takes 6 hours for a ship to go back and forth between the two ports. It travels 10 kilometers more per hour when it goes out than when it comes back. Therefore, it travels 25 kilometers more in the first 3 hours than in the last 3 hours. How many kilometers is the distance between the two ports? ．
一只船甲乙两港间往返一次共用6小时，去时顺水比回来时每小时多行10千米，因此前3小时比后3小时多行25千米，两港间的路程是多少千米．
<gadget id="calculator">25 / 10</gadget>
<output>5/2 = around 2.5</output>

<gadget id="calculator">6 - (5/2)</gadget>
<output>7/2 = around 3.5</output>

<gadget id="calculator">(7/2) - (5/2)</gadget>
<output>1</output>

<gadget id="calculator">25 / 1</gadget>
<output>25</output>

<gadget id="calculator">25 * (7/2)</gadget>
<output>175/2 = around 87.5</output>

<result>175/2 = around 87.5</result>