There are several numbers whose average is 20. After reducing one of the numbers by 100, the average becomes 15. How many numbers are there in total?
有若干个数的平均数为20，把其中一个数减少100后，其平均数变为15，求共有多少个数？
<gadget id="calculator">20 - 15</gadget>
<output>5</output>

<gadget id="calculator">100 / 5</gadget>
<output>20</output>

<result>20</result>