There are four numbers: 2, 3, 0, 7, first add a number to increase their average by 1, then what is the added number.
有四个数：2，3，0，7，先要添加一个数，使它们的平均数增加1，则添加的数是多少．
<gadget id="calculator">2 + 3 + 0 + 7</gadget>
<output>12</output>

<gadget id="calculator">12 / 4</gadget>
<output>3</output>

<gadget id="calculator">3 + 1</gadget>
<output>4</output>

<gadget id="calculator">4 + 1</gadget>
<output>5</output>

<gadget id="calculator">4 * 5</gadget>
<output>20</output>

<gadget id="calculator">20 - 12</gadget>
<output>8</output>

<result>8</result>