The average of the four numbers A, B, C, and D is 20. If one of the numbers is changed to 30, the average of these four numbers is 25. What is the original number?
甲、乙、丙、丁四个数的平均数为20，若把其中一个数改为30，则这四个数的平均数为25，这个数原来是多少．
<gadget id="calculator">25 * 4</gadget>
<output>100</output>

<gadget id="calculator">20 * 4</gadget>
<output>80</output>

<gadget id="calculator">100 - 80</gadget>
<output>20</output>

<gadget id="calculator">30 - 20</gadget>
<output>10</output>

<result>10</result>