City code says you need to have a total of at least 54 toilet stalls, but each bathroom can only have 6 stalls.
So, Let the number of bathrooms to be built be = x
Number of toilet stalls needed = 54
Number of stalls each bathroom can have = 6
So, the number of bathrooms to be built are =



Hence, one must build 9 bathrooms with 6 stalls each.