A truck delivers goods to the city. It travels 68 kilometers per hour when it goes there. It takes 3 hours to travel, and it takes 4 hours to return. How many kilometers per hour does the truck travel on average when it returns?
一辆货车往城里送货，去时每小时行68千米，行了3小时，返回时用了4小时，返回时货车平均每小时行多少千米？
<gadget id="calculator">3 / 4</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">68 * (3/4)</gadget>
<output>51</output>

<result>51</result>