The express train and the slow train leave oppositely from places A and B at the same time. It is known that the express train travels 40 kilometers per hour. After 4 hours, the express train has passed the midpoint of 34 kilometers. At this time, the distance between the slow train and the slow train is 7 kilometers. How many kilometers per hour?
快车和慢车同时从甲、乙两地相对开出，已知快车每小时行40千米，经过4小时，快车已驶过中点34千米，这时与慢车还相距7千米，慢车每小时行多少千米？
<gadget id="calculator">40 * 4</gadget>
<output>160</output>

<gadget id="calculator">160 - 34</gadget>
<output>126</output>

<gadget id="calculator">34 + 7</gadget>
<output>41</output>

<gadget id="calculator">126 - 41</gadget>
<output>85</output>

<gadget id="calculator">85 / 4</gadget>
<output>85/4 = around 21.25</output>

<result>85/4 = around 21.25</result>