Uncle Li drove a farm vehicle to the city to buy chemical fertilizers. When he went there, he traveled 40 kilometers per hour. It took him 3 hours to get to the city and 4 hours when he came back. How many kilometers per hour did he drive on average when he came back?
李大爷开农用车到城里买化肥，去的时候每小时是40千米，用了3个小时到城里，回来时用了4小时，回来时平均每小时行驶多少千米？
<gadget id="calculator">3 / 4</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">40 * (3/4)</gadget>
<output>30</output>

<result>30</result>