A ship transports goods from A to B, and it takes 7 hours to sail there. The speed when returning was 35 km/h, and it took only 6 hours. May I ask how many kilometers per hour the freighter travels on average?
一艘轮船从甲地运输货物到乙地，去时航行用了7小时．返回时的速度是35千米/时，只用了6小时．请问去时货轮平均每小时行多少千米？
<gadget id="calculator">6 / 7</gadget>
<output>6/7 = around 0.857143</output>

<gadget id="calculator">35 * (6/7)</gadget>
<output>30</output>

<result>30</result>