They tell you that the average (16+12+X)/3 is between 19 and 25 inclusive, so we write it like that with x the number of starfish Carey bought:

19<= (28+x)/3 <= 25

They ask you the biggest difference which is possible between the numbers of starfish Carey bought.

The smallest value Carey bought is 19=(28+x)/3
The highest value Carey bought is 25=(28+x)/3
It gives you x=29 and x=47

And the result is 47-29=18

Did you guess for your answer 22?