On a map with a scale of 1:30000000, the total length of the Yangtze River is measured to be 21 centimeters. How many kilometers is the actual length of the Yangtze River?
在一幅比例尺为1：30000000的地图上，量得长江的全长是21厘米．长江实际的全长大约是多少千米？
<gadget id="calculator">1 / 30_000_000</gadget>
<output>1/30_000_000 = around 0</output>

<gadget id="calculator">21 / (1/30_000_000) / 100_000</gadget>
<output>6_300</output>

<result>6_300</result>