The average of seven numbers is 2.9. Put these seven numbers in a row. The average of the first three numbers is 2.5, and the average of the last five numbers is 3.2. What is the third number?
七个数的平均数是2.9，把这七个数排成一排，前三个数的平均数是2.5，后五个数的平均数是3.2，第三个数是几？
<gadget id="calculator">2.5 * 3</gadget>
<output>7.5</output>

<gadget id="calculator">3.2 * 5</gadget>
<output>16</output>

<gadget id="calculator">2.9 * 7</gadget>
<output>20.3</output>

<gadget id="calculator">16 - 20.3</gadget>
<output>-4.3</output>

<gadget id="calculator">7.5 + (-4.3)</gadget>
<output>3.2</output>

<result>3.2</result>