7 out of 14 contestants did last time, which is 1/2 of the contestants. You would assume that the same proportion would qualify when there are 10 contestants, so you would assume that 1/2 of 10 would also qualify for the bonus round.
Domain represents all possible x-values which can be used in the function. Since x gives the number of days the home was rented, you can only have a positive number of days.