The teacher wrote several consecutive natural numbers starting from 1, 1, 2, 3, ... on the blackboard, and then erased one of the numbers. The average of the remaining numbers is 10.8. What is the number of natural numbers erased?
老师在黑板上写了从1开始的若干个连续自然数1，2，3，…，后来擦掉其中一个数，剩下的数的平均数是10.8，擦掉的自然数是多少．
<gadget id="calculator">22 * 10</gadget>
<output>220</output>

<gadget id="calculator">220 + 11</gadget>
<output>231</output>

<gadget id="calculator">20 * 10.8</gadget>
<output>216</output>

<gadget id="calculator">231 - 216</gadget>
<output>15</output>

<result>15</result>