1, 1 species is attached to a carbon of a three-membered ring in the form of ethyl.
2. There are 1 species attached to a carbon of a three-membered ring in the form of two methyl groups.
3. When stereoisomerism is considered:
(1) Two carbons are connected to two carbons of a three-membered ring in the form of two methyl groups. Because the orbital hybridization type of carbon on the ternary ring is sp3 hybridization (the connected tetravalent bonds are similar to regular tetrahedron structure), these two carbons will point to the upper or lower side of the plane determined by the ternary ring.
(2) 1 species if they point in the same direction at the same time.
(3) pointing in two directions at the same time, there are two kinds:
Suppose that three carbons on the ternary ring are labeled as NO. They are 1, No.2 and No.3 respectively. If there is no methyl group. 1 carbon faces upwards, the methyl group attached to No.2 carbon faces downwards, and the methyl group attached to No.3 carbon faces downwards. There are two different situations (mirror isomerization). It's not easy to understand You can try making a model. In any case, it is impossible to make the two coincide.
Mathematical problems, assuming that z is I, are solved by special methods.