In a series of numbers, if the next number is different from the previous number by the same number, we call this series an arithmetic progression. For example: 3, 7, 11, 15, 19... There are 5 numbers 2.5, A, □, C, 10.9 to form an arithmetic sequence, then what is A=?
在一列数中，若后一个数都比相邻的前一个数相差同一个数，我们称这列数为等差数列．比如：3，7，11，15，19…现有5个数2.5，A，□，C，10.9组成等差数列，那么A=多少．
<gadget id="calculator">10.9 - 2.5</gadget>
<output>8.4</output>

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

<gadget id="calculator">8.4 / 4</gadget>
<output>2.1</output>

<gadget id="calculator">2.5 + 2.1</gadget>
<output>4.6</output>

<result>4.6</result>