Li Hong went to Nanjing with her parents for fun. The speed of the car was 90 km/h, and it took 5 hours. It took 4 hours to return to the original road. What is the average speed of the car?
李红同学和父母一起去南京游玩，去的时候汽车的速度是90千米/时，用了5小时，原路返回用了4小时，汽车往返的平均速度是多少？
<gadget id="calculator">90 * 5 * 2</gadget>
<output>900</output>

<gadget id="calculator">5 + 4</gadget>
<output>9</output>

<gadget id="calculator">900 / 9</gadget>
<output>100</output>

<result>100</result>