The following column of numbers is arranged according to certain rules: 3, 12, 21, 30, 39, 48, 57..., then what is the 21st number in this column of numbers?
下面一列数是按一定规律排列的：3、12、21、30、39、48、57…，那么这列数的第21个数是多少．
<gadget id="calculator">3 / 3</gadget>
<output>1</output>

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

<gadget id="calculator">20 * 3</gadget>
<output>60</output>

<gadget id="calculator">1 + 60</gadget>
<output>61</output>

<gadget id="calculator">61 * 3</gadget>
<output>183</output>

<result>183</result>