There are 5 numbers arranged from left to right. The sum of these 5 numbers is 27.6, the sum of the first 3 numbers is 20.5, and the sum of the last 3 numbers is 9.1. What is the middle number?
从左到右排列着5个数，这5个数的和是27.6，前3个数的和是20.5，后3个数的和是9.1，中间一个数是多少？
<gadget id="calculator">9.1 - 27.6</gadget>
<output>-18.5</output>

<gadget id="calculator">20.5 + (-18.5)</gadget>
<output>2</output>

<result>2</result>