The average of five numbers is 20. If one of the numbers is changed to 40, the average is 25. What is the changed number?
五个数的平均数是20，若把其中一个数改为40，则平均数是25，这个改动的数是多少．
<gadget id="calculator">25 * 5</gadget>
<output>125</output>

<gadget id="calculator">20 * 5</gadget>
<output>100</output>

<gadget id="calculator">125 - 100</gadget>
<output>25</output>

<gadget id="calculator">40 - 25</gadget>
<output>15</output>

<result>15</result>