From station A to station B, it takes 4 hours for car A to travel, and 5 hours for car B to travel. If the two cars depart from stations A and B at the same time, when the untraveled distance is the entire distance (1/10) , How many hours did the two cars go together?
从甲站到乙站，甲车要行4小时，乙车要行5小时，如果两车同时分别从甲、乙两站相对开出，当未行的路程是全程的(1/10)时，两车行了多少小时？
<gadget id="calculator">1 / 10</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">1 - (1/10)</gadget>
<output>9/10 = around 0.9</output>

<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">(9/10) / (9/20)</gadget>
<output>2</output>

<result>2</result>