The postman Xiao Li delivers a letter from A to B. He travels 20km per hour when he goes there, which takes 7.5 hours. He walks 50km per hour when he returns. How many hours can he get back to A? (Solution with proportional knowledge)
邮递员小李从A地到B地送信，去时每小时走20km，用可7.5小时，回的时候每小时走50km，多小时可以回到A地？(用比例知识解)
<gadget id="calculator">20 * 7.5</gadget>
<output>150</output>

<gadget id="calculator">150 / 50</gadget>
<output>3</output>

<result>3</result>