Hi anyone knows how to solve this?

i) Consider Z*_{15}, the group of invertible element of Z_{15}under the operation of multiplication (mod 15). What is the size of Z*_{15}?

ii) Consider Z*_{6}, the group of invertible element of Z_{6}under the operation of multiplication (mod 6). What is the size of Z*_{6}?

Appreciate if anyone can help me (: