The average of 5 numbers is 70. If one of the numbers is changed to 90, the average of these 5 numbers is 80. What is the original number that has been changed?
某5个数的平均数是70，若把其中的一个数改为90，则这5个数的平均数是80．改的这个数原来是多少．
<gadget id="calculator">80 - 70</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>