The fast train and the slow train drive from point A to point B at the same time. After 2 hours, the fast train is 40 kilometers ahead of the slow train. The speed of the fast train is 100 km/h. Find the speed of the slow train.
快车和慢车同时从A点开往B点，2小时后，快车比慢车领先40千米，快车速度是100千米/时，求慢车速度．
<gadget id="calculator">100 * 2</gadget>
<output>200</output>

<gadget id="calculator">200 - 40</gadget>
<output>160</output>

<gadget id="calculator">160 / 2</gadget>
<output>80</output>

<result>80</result>