What this is saying is...20 is a whole number; but 3/4 is not. What they are saying is, if you have to add a mixed number with a whole number, you can add both of the whole numbers together. Then, you add the 3/4 at the end...because it is not a whole number. You can break apart a mixed number by separating the whole number and the fraction.
Answer (13 of the original arrangements are duplicates now, as
AAAF F F FDDDDCC, AAF F F FDDDDCCA, ..., CAAAF F F FDDDDC
are identical): 900900/13 = 69300.
