Scott is curious as to whether or not the students at three different schools are equally smart. He collects a sample of 9 students from Wilson High School, 9 students from Lakewood HS, 9 from Millikan HS and give same tests. The average for Wilson was 75, Lakewood was 77, Millikan was 88. (The average for all 27 students was 80) The SS(Within)=1920. Calculate F-Stat

What I have so far:

F = (882/2) / (1920/24) = 5.5125 > 3.40 (Critical Value) , therefore, reject Null.

However, I do not understand how to obtain denominator of 1920, which is 24. I need help on how to obtain number 24 above.