Answer:There you go X is the middle, a only brother, b only sister.
a X =11
b X =9
a b X =14 (20-6)
a b 2x=20
X=6
So 6 in the middle, 6 on the outside, 5 only brothers, 3 only sisters. let me know if I missed anything but we have a total of 20, 6 with none, 11 with brothers and 9 with sisters
Explanation: