The average of five numbers is 20. After changing one of the numbers to 8, the average of these five numbers becomes 17. What is the original number of this change?
五个数的平均数是20，把其中一个数改为8后，这五个数的平均数变成了17，这个改动的数原来为多少．
<gadget id="calculator">20 * 5</gadget>
<output>100</output>

<gadget id="calculator">17 * 5</gadget>
<output>85</output>

<gadget id="calculator">100 - 85</gadget>
<output>15</output>

<gadget id="calculator">8 + 15</gadget>
<output>23</output>

<result>23</result>