A certain car can run for up to 9 hours with a full fuel tank. It is known that the car travels 90 kilometers per hour when it goes out and 72 kilometers per hour when it returns. How many hours does this car have to go before returning?
某辆汽车油箱加满油最多能行9小时，已知汽车开出去每小时行90千米，返回时每小时行72千米，这辆汽车最多行多少小时就必须返回？
<gadget id="calculator">1 / 90</gadget>
<output>1/90 = around 0.011111</output>

<gadget id="calculator">1 / 72</gadget>
<output>1/72 = around 0.013889</output>

<gadget id="calculator">(1/90) + (1/72)</gadget>
<output>1/40 = around 0.025</output>

<gadget id="calculator">9 / (1/40)</gadget>
<output>360</output>

<result>360</result>