well you have to find out how many he can buy maximum. Then it would be from 1 to whatever number that is.
To find the maximum you divide 300 by 49 which equals a little over 6
so because you can but a little of a ceiling fan you can only buy 6 fans.
So in context to the problem your domain would be {c|1<x<6}
Does that make sense? If not, feel free to ask me in comments :)