On a map with a scale of 1:500,000, the distance from Nan'an to Quanzhou on the map is 4.2 cm. What is the actual distance from Nan'an to Quanzhou in kilometers?
在一幅比例尺是1：500000的地图上，量得南安至泉州的图上距离是4.2厘米，南安至泉州的实际距离是多少千米．
<gadget id="calculator">1 / 500_000</gadget>
<output>1/500_000 = around 0.000002</output>

<gadget id="calculator">4.2 / (1/500_000) / 100_000</gadget>
<output>21</output>

<result>21</result>