Five numbers with an average of 12.7 are arranged in a row. The average of the first three numbers is 12.8, and the average of the last three numbers is 12.4. What is the third number?
平均数是12.7的五个数排成一列，前三个数的平均数是12.8，后三个数的平均数是12.4，那么第三个数是多少？
<gadget id="calculator">12.8 * 3</gadget>
<output>38.4</output>

<gadget id="calculator">12.4 * 3</gadget>
<output>37.2</output>

<gadget id="calculator">12.7 * 5</gadget>
<output>63.5</output>

<gadget id="calculator">37.2 - 63.5</gadget>
<output>-26.3</output>

<gadget id="calculator">38.4 + (-26.3)</gadget>
<output>12.1</output>

<result>12.1</result>