Well she starts with 24 buns she ate 1/2 half of a bun leaving 23 1/2 buns left then she gave here mother and father a bun each so you do 23 1/2 - 2 which gets you 21 1/2 buns. So the total number of buns that she has left is 21 and 1/2 buns.
Hope i helped:)