There are 8 planes parked at the airport. After the first plane takes off, another plane takes off every 4 minutes. 2 minutes after the first plane took off, a plane landed on the airport, and every 6 minutes thereafter, a plane landed on the airport, and the planes that landed on the airport followed the original 8 planes every 4 minutes take off. How long has it been since the first plane took off without any planes staying at the airport?
机场上停着8架飞机，第一架飞机起飞后，每隔4分钟，便有一架飞机接着起飞．在第一架飞机起飞后2分钟，有一架飞机在机场上降落，此后每隔6分钟，有一架飞机在机场上降落，降落在机场上的飞机依次每隔4分钟在原有的8架飞机之后起飞．从第一架飞机起飞以后，经过多长时间，机场上没有飞机停留？
<gadget id="calculator">12 * 6</gadget>
<output>72</output>

<gadget id="calculator">8 + 72 + 4</gadget>
<output>84</output>

<result>84</result>