There are 4 numbers: 2, (1/5), 0, 1.2, now we want to add a number to increase their average number by 1, what is the added number.
有4个数：2，(1/5)，0，1.2，现要添加一个数，使他们的平均数增加1，则添加的数是多少．
<gadget id="calculator">3.4 / 4</gadget>
<output>0.85</output>

<gadget id="calculator">0.85 + 1</gadget>
<output>1.85</output>

<gadget id="calculator">1.85 * 5</gadget>
<output>9.25</output>

<gadget id="calculator">9.25 - 3.4</gadget>
<output>5.85</output>

<result>5.85</result>