Question

how many methylene group units ( -CH2- ) are present in propane ?

Answer 1

CH3-CH2-CH3 is formula for propane here three units are present

Answer 2

The structure of propane is:

CH3CH2CH3

We can clearly see that ther in one meyhylene group (CH2) in propane.

For further determining of number of methylene groups first write the structure of the compound and then find how many CH2 groups are present.