The boys need to arranged like this:

_B_B_B_B_B_B_B_B_B_B_

and the remaining 15 girls have to go into the 11 blank spots. The first or last blank could be left empty, but there has to be at least one girl in each of the 9 blank spots between the boys

_BG_BG_BG_BG_BG_BG_BG_BG_BG_BG_

That leaves 6 girls that can go into any of the 11 spots.

The number of different ways to distribute n indistinguishable girls into k distinguishable spots is