Suppose mindy's age is 9.
If the average age of the rest of her friends is 9,
Average age including mindy would be 9.

However, due to mindy's age being 27 the average age of mindy n friends increases to 11.

As such it can be said that the 18 year difference between 9 and 27 is the cause of the 2 year increase in average age from 9 to 11.

Spread this 18 year difference among mindy and friends, and it can be deduced that 18 would have to be split among 9 people (18÷2=9) to achieve an average age of 11.

Thus, counting mindy, there are a total of 9 people in the group of friends, so mindy has 9-1= 8 friends.

Working:
11-9=2

27-9=18

18÷2=9

9-1=8