There are 5 boys and 4 girls in my class. All of them are distinguishable.
In how many ways can they be seated in a row of 9 chairs such that at least 3 girls are all next to each other?
+118608 ok
how many ways can the 3 girls sit next to each other ? 4P3 = 24
Now lets take just one of these 24 permutations and put them together with the others.
I will take the 3 girls as one (siamese triplets) + one more girl +5boys = 7 so 7! permutations.
So altogether I think that there will be 24*7! permutations
$${\mathtt{24}}{\mathtt{\,\times\,}}{\mathtt{7}}{!} = {\mathtt{120\,960}}$$ 
I think that is correct.
Having that 4th girl is throwing me a bit, but I think it is ok.
+118608 ok
how many ways can the 3 girls sit next to each other ? 4P3 = 24
Now lets take just one of these 24 permutations and put them together with the others.
I will take the 3 girls as one (siamese triplets) + one more girl +5boys = 7 so 7! permutations.
So altogether I think that there will be 24*7! permutations
$${\mathtt{24}}{\mathtt{\,\times\,}}{\mathtt{7}}{!} = {\mathtt{120\,960}}$$ I think that is correct.
Having that 4th girl is throwing me a bit, but I think it is ok.
