A construction team installed a gas pipeline, and installed 840 meters in the first 6 days. According to this calculation, it took 8 days to complete the installation. How long is this gas pipeline in meters? (Solution by two methods, one of which should be solved by proportion)
一个施工队安装一条煤气管道，头6天装了840米，照这样计算，共用8天全部安装完毕．这条煤气管道全长多少米？(用两种方法解，其中一种要用比例解)
<gadget id="calculator">840 / 6</gadget>
<output>140</output>

<gadget id="calculator">140 * 6</gadget>
<output>840</output>

<gadget id="calculator">140 * 4</gadget>
<output>560</output>

<gadget id="calculator">840 + 560</gadget>
<output>1_400</output>

<result>1_400</result>