There are five numbers in a row, their average is 28, the average of the first three numbers is 24, and the average of the last three numbers is 25, what is the number in the middle?
有五个数排成一列，它们的平均数是28，前三个数的平均数是24，后三个数的平均数是25，中间的那个数是多少．
<gadget id="calculator">24 * 3</gadget>
<output>72</output>

<gadget id="calculator">25 * 3</gadget>
<output>75</output>

<gadget id="calculator">28 * 5</gadget>
<output>140</output>

<gadget id="calculator">75 - 140</gadget>
<output>-65</output>

<gadget id="calculator">72 + (-65)</gadget>
<output>7</output>

<result>7</result>