The second item of an arithmetic sequence is 2.8, and the third item is 3.1. What is the 15th item of this arithmetic sequence?
一个等差数列的第2项是2.8，第三项是3.1，这个等差数列的第15项是多少．
<gadget id="calculator">3.1 - 2.8</gadget>
<output>0.3</output>

<gadget id="calculator">2.8 - 0.3</gadget>
<output>2.5</output>

<gadget id="calculator">15 - 1</gadget>
<output>14</output>

<gadget id="calculator">14 * 0.3</gadget>
<output>4.2</output>

<gadget id="calculator">2.5 + 4.2</gadget>
<output>6.7</output>

<result>6.7</result>