How many factors does 1155 have that are divisible by 3?

Explanation

`:.` In order to be a factor of 1155 divisible by 3, we have to choose one or two of 5,7,11 along with 3 or 3 alone.
`:." The number of ways." = ""^3C_1+ ""^3C_2 + ""^3C_0 =2^3-1 =7`
` :.' The number of factors is 7 excluding 1155 itself.