It takes 9 hours for a car to drive from point A to point B, and it takes 10 hours for another car to drive from point B to point A. Two cars drive towards each other from two places at the same time, and after how many hours the two cars meet?
一辆汽车从甲地开往乙地需要9小时，另一辆此车从乙地开往甲地需10小时．两辆汽车同时从两地相向开出，经过几小时两车相遇？
<gadget id="calculator">1 / 9</gadget>
<output>1/9 = around 0.111111</output>

<gadget id="calculator">1 / 10</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">(1/9) + (1/10)</gadget>
<output>19/90 = around 0.211111</output>

<gadget id="calculator">1 / (19/90)</gadget>
<output>90/19 = around 4.736842</output>

<result>90/19 = around 4.736842</result>