Find the next number in the given number series? 11, 5.5, 10, 6.5, 9, ?

The given number series follows a pattern that

11

11 - 5.5 = 5.5

5.5 + 4.5 = 10

10 - 3.5 = 6.5

6.5 + 2.5 = 9

9 - 1.5 = 7.5

Hence, the next number in the given number series is 7.5.