The length of Beijing Metro Line 1 from Pingguoyuan Station to Sihuidong Station is about 7.8 cm in the scale of 1:400000. How many kilometers is the actual length from Pingguoyuan Station to Sihuidong Station?
北京地铁1号线从苹果园站至四惠东站，在比例尺是1：400000的图中长度大约是7.8厘米，从苹果园站至四惠东站的实际长度大约是多少千米？
<gadget id="calculator">1 / 400_000</gadget>
<output>1/400_000 = around 0.000002</output>

<gadget id="calculator">7.8 / (1/400_000) / 100_000</gadget>
<output>31.2</output>

<result>31.2</result>