The average of the three numbers is 20, the average after adding the fourth number is 30, what is the fourth number
三个数的平均数是20，加上第四个数后的平均数是30，第四个数是多少
<gadget id="calculator">30 * 4</gadget>
<output>120</output>

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

<gadget id="calculator">120 - 60</gadget>
<output>60</output>

<result>60</result>