The average of 5 numbers is 70, if one of them is changed to 90, the average of 5 numbers becomes 80. What was the number before the change?
5个数的平均数是70，若把其中一个改为90，5个数的平均数变成80．改动前这个数是多少？
<gadget id="calculator">90 - 80</gadget>
<output>10</output>

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

<gadget id="calculator">90 - 50</gadget>
<output>40</output>

<result>40</result>