He will need 180 square tiles because 12 inches is a foot. So 15 feet times 12 feet is 180 feet of space that you need to cover with one foot tiles :P
Not addition, since

and 0 doesn't belong to the set.
Not subtraction, since

.
It is closed under multiplication. The set appears to contain all non-zero multiples of 3.
Not division, since dividing some of the numbers results in rationals which do not belong to the set.
What other operations are you supposed to consider?
10 maybe I’m not sure but if this is a question from your teacher then ask them what the f&&&&
5th term? i’m pretty sure