The Mid-Autumn Festival is here, red and green lanterns are hung on the school gate, a total of 21 are hung from the beginning to the end, one red light is hung every 30 decimeters, and one red light is hung between two adjacent red lights. Only green light, ask how wide is the gate of the school?
中秋节到了，学校大门上挂了红绿两种颜色的彩灯，从头到尾一共挂了21只，每隔30分米挂1只红灯，相邻的2只红灯之间挂了1只绿灯，问学校的大门有多宽？
<gadget id="calculator">21 - 1</gadget>
<output>20</output>

<gadget id="calculator">20 / 2</gadget>
<output>10</output>

<gadget id="calculator">30 * 10</gadget>
<output>300</output>

<result>300</result>