The express train and the local train depart from the east and west oppositely at the same time. It is known that the express train travels 80 kilometers per hour. After 3 hours, the express train has passed the midpoint of 30 kilometers, and the distance between the express train and the slow train is still 12 kilometers. How many kilometers per hour does the slow train travel.
快车和慢车同时从东西两地相对开出．已知快车每小时行80千米，经过3小时后，快车已驶过中点30千米，这时与慢车还相距12千米．慢车每小时行多少千米．
<gadget id="calculator">80 * 3</gadget>
<output>240</output>

<gadget id="calculator">240 - 30</gadget>
<output>210</output>

<gadget id="calculator">30 + 12</gadget>
<output>42</output>

<gadget id="calculator">210 - 42</gadget>
<output>168</output>

<gadget id="calculator">168 / 3</gadget>
<output>56</output>

<result>56</result>