person drives a car at a speed of 20 kilometers per hour for 90 kilometers, and returns at 30 kilometers per hour, what is the average speed of the whole journey?
某人驾车以每小时20千米的速度行了90千米，返回时每小时行30千米，其全程的平均速度是多少？
<gadget id="calculator">90 + 90</gadget>
<output>180</output>

<gadget id="calculator">90 / 20</gadget>
<output>9/2 = around 4.5</output>

<gadget id="calculator">90 / 30</gadget>
<output>3</output>

<gadget id="calculator">(9/2) + 3</gadget>
<output>15/2 = around 7.5</output>

<gadget id="calculator">180 / (15/2)</gadget>
<output>24</output>

<result>24</result>