A train of passenger trains and a train of freight trains start from two places at the same time and travel towards each other. It takes 4 hours for the passenger train to complete the journey, and 5 hours for the complete journey of the freight train. The passenger train travels 20 kilometers more per hour than the freight train. How many miles does the truck travel per hour?
一列客车和一列货车同时从两地出发相向而行，客车行完全程要4小时，货车行完全程要5小时，客车比货车每小时多行20千米．货车每小时行多少千米？
<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">(1/4) - (1/5)</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">20 / (1/20) / 5</gadget>
<output>80</output>

<result>80</result>