The average of four numbers is 8. If one of the numbers is changed to 1, the average of these four numbers is 6. What is the original number?
有四个数的平均数是8，若把其中一个数改为1，则这四个数的平均数是6，这个数原是多少．
<gadget id="calculator">8 - 6</gadget>
<output>2</output>

<gadget id="calculator">2 * 4</gadget>
<output>8</output>

<gadget id="calculator">8 + 1</gadget>
<output>9</output>

<result>9</result>