My mother is going to take Doudou to Shanghai to watch the Expo this summer by train. On a map with a scale of 1:2,000,000, the length of the railway line from Wuhu to Shanghai is measured to be 21 centimeters, and the train travels about 120 kilometers per hour. How long will it take for them to take the train?
妈妈今年暑假准备带豆豆乘火车到上海看世博．在比例尺是1：2000000的地图上，量得芜湖到上海的铁路线长21厘米，火车每小时行驶约120千米，她们要坐多长时间的火车？
<gadget id="calculator">1 / 2_000_000</gadget>
<output>1/2_000_000 = around 0</output>

<gadget id="calculator">21 / (1/2_000_000) / 100_000 / 120</gadget>
<output>7/2 = around 3.5</output>

<result>7/2 = around 3.5</result>