yes she is correct..
the reason why is because if she gives her horse 2 apples a day for 10 days, she will have spent 20 apples.
and so to get the number of apples she has remaining after 10 days, just subtract 20 from 24:
24-20 = 4
and you get four. That means that after 10 days she will indeed have 4 apples left