The express train and the slow train are running towards each other from places A and B at the same time. The express train travels 40 kilometers per hour. After 3 hours, the express train has passed the midpoint of 25 kilometers. At this time, the distance between the express train and the slow train is still 7 kilometers. How many kilometers per hour does the slow car travel?
快车和慢车同时从甲、乙两地相向而行，快车每小时行40千米，经过3小时，快车已驶过中点25千米，这时快车与慢车还相距7千米．慢车每小时行多少千米？
<gadget id="calculator">40 * 3</gadget>
<output>120</output>

<gadget id="calculator">120 - 25</gadget>
<output>95</output>

<gadget id="calculator">25 + 7</gadget>
<output>32</output>

<gadget id="calculator">95 - 32</gadget>
<output>63</output>

<gadget id="calculator">63 / 3</gadget>
<output>21</output>

<result>21</result>