There are 81 small flags planted on the side of the runway, and their intervals are 2 meters. Now it is necessary to insert only 21 small flags (flags at both ends do not move), how many meters should the interval be changed?
跑道一旁插着81面小旗，它们的间隔是2米．现在要改为只插21面小旗(两端的旗子不动)，间隔应改为多少米
<gadget id="calculator">81 - 1</gadget>
<output>80</output>

<gadget id="calculator">80 * 2</gadget>
<output>160</output>

<gadget id="calculator">21 - 1</gadget>
<output>20</output>

<gadget id="calculator">160 / 20</gadget>
<output>8</output>

<result>8</result>