ANSWER:
C = {9, 18, 27, 36, 45, 54,63, 72}
SOLUTION:
C = {n} where n are multiples of 9 less than 81
The multiples of 9 are 9, 18, 27, 36, 45, 54, 63, 72, 81, so on and so forth
Just remove the multiples that are 81 and more
C = {9, 18, 27, 36, 45, 54,63, 72, 81} << remove 81
C = {9, 18, 27, 36, 45, 54, 63, 72}
