The average of 5 numbers is 20. If one number is changed to 4, the average becomes 18. What is the original number?
5个数的平均数是20，将一个数改为4，平均数就变成了18，这个数原来是多少．
<gadget id="calculator">20 * 5</gadget>
<output>100</output>

<gadget id="calculator">18 * 5</gadget>
<output>90</output>

<gadget id="calculator">100 - 90</gadget>
<output>10</output>

<gadget id="calculator">10 + 4</gadget>
<output>14</output>

<result>14</result>