Uncle Wang departs from the county seat to Wangzhuang Township to deliver fertilizers. If he travels 40 kilometers per hour, he can arrive there in 3 hours. When he returns, he takes 1 hour longer than when he went there. How many kilometers slower per hour on average when he returns than when he went there?
王叔叔从县城出发去王庄乡送化肥，去时如果每小时行40千米，3小时可以到达，返回时比去时多用1小时，返回时平均每小时比去时慢多少千米？
<gadget id="calculator">3 + 1</gadget>
<output>4</output>

<gadget id="calculator">3 / 4</gadget>
<output>3/4 = around 0.75</output>

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

<gadget id="calculator">40 - 30</gadget>
<output>10</output>

<result>10</result>