A car departs from A to B to deliver goods. When going there, it travels 40km per hour. When returning, it travels 20km more per hour because the car is empty. It takes 7.5 hours for a round trip. Find the distance between A and B.
一辆汽车从A地出发去B地送货，去时每时行40km，返回时因空车每时多行20km，往返一次共用7.5时，求A，B两地的距离．
<gadget id="calculator">40 * 7.5</gadget>
<output>300</output>

<gadget id="calculator">20 * 7.5</gadget>
<output>150</output>

<gadget id="calculator">300 + 150</gadget>
<output>450</output>

<gadget id="calculator">40 + 40 + 20</gadget>
<output>100</output>

<gadget id="calculator">450 / 100</gadget>
<output>9/2 = around 4.5</output>

<gadget id="calculator">40 * (9/2)</gadget>
<output>180</output>

<result>180</result>