The average of five numbers is 60. If one of the numbers is changed to 80, the average becomes 70. What is the original number that was changed?
有五个数的平均数是60，如果把其中一个数改为80，平均数就变成70．被改动的这个数原来是多少．
<gadget id="calculator">70 - 60</gadget>
<output>10</output>

<gadget id="calculator">10 * 5</gadget>
<output>50</output>

<gadget id="calculator">80 - 50</gadget>
<output>30</output>

<result>30</result>