It takes 4 hours for a passenger car to travel from city A to city B, and 5 hours for a truck to travel from city B to city A. How many hours after the two vehicles depart from city A and city B respectively at the same time, they meet.
客车从甲城市到乙城市要4小时，货车从乙城市到甲城市行驶5小时，两车同时分别从甲城市和乙城市出发多少小时后相遇．
<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>9/20 = around 0.45</output>

<gadget id="calculator">1 / (9/20)</gadget>
<output>20/9 = around 2.222222</output>

<result>20/9 = around 2.222222</result>