There are 5 numbers whose average value is 21, remove one of the numbers, and the remaining 4 numbers have an average value of 20, what is the removed number?
有5个数的平均值为21，去掉其中一个数，余下的4个数的平均值为20，则去掉的这个数为多少．
<gadget id="calculator">21 * 5</gadget>
<output>105</output>

<gadget id="calculator">20 * 4</gadget>
<output>80</output>

<gadget id="calculator">105 - 80</gadget>
<output>25</output>

<result>25</result>