If 4 boys and 5 girls are seated in a row of 9 chairs, what is the probability that the 2 girls will sit next to each other?
Which 2 girls? Any two girls ?
Can 3 girls or more sit beside each other, or only 2 girls?
The wording is not specific enough.
I will assume that we are talking about 2 specific girls, and instead of 9 chairs we have to arrange them in 8 ways.
number of ways = 8!
(suppose the 2 specific girls are one "group", so we are arranging the 4 boys, the "group" and 3 other girls, so arrange 8 )
But our two girls AB could be arranged as BA
number of ways to seat them = 2 x 8! or 80640