The general molecular formula of an alkane is written as C_nH_2n+2, and the general formula of an alkene is written as C_nH_2n where ‘n’ is the number of carbon atoms.

Now for n=20, the molecular formula of

a) alkane will be C₂₀H_2x20+2 or C₂₀H₄₂

b) alkene will be C₂₀H_2x20 or C₂₀H₄₀