The average of three numbers is 23, two of which are 20 and 30 respectively, what is the other number.
三个数的平均数是23，其中的两个数分别是20和30，另一个数是多少．
<gadget id="calculator">23 * 3</gadget>
<output>69</output>

<gadget id="calculator">69 - 20 - 30</gadget>
<output>19</output>

<result>19</result>