A batch of goods is completed in 4 hours by car A alone, 5 hours by car B alone, and 2 hours after the two cars are combined, and the rest is transported by car B. How many hours will it take?
一批货物，甲车单独运4小时完成，乙车单独运5小时完成，两车合运2小时后，余下由乙车运，还需多少小时可以运完？
<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">(9/20) * 2</gadget>
<output>9/10 = around 0.9</output>

<gadget id="calculator">1 - (9/10)</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">(1/10) / (1/5)</gadget>
<output>1/2 = around 0.5</output>

<result>1/2 = around 0.5</result>