The average of 5 numbers is 5. If one of the numbers is changed to 2, the average of these 5 numbers is 4. What is the changed number?
有5个数的平均数是5，如果把其中一个数改为2，这5个数的平均数为4，这个被改动的数原来是几？
<gadget id="calculator">5 * 5</gadget>
<output>25</output>

<gadget id="calculator">4 * 5</gadget>
<output>20</output>

<gadget id="calculator">25 - 20</gadget>
<output>5</output>

<gadget id="calculator">2 + 5</gadget>
<output>7</output>

<result>7</result>