The ratio of number of carbon atoms and number of hydrogen atoms present in an alkene is

The general formula of alkanes is ${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2\mathrm{n}+2}$

The general formula of alkenes is ${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2\mathrm{n}}$

The general formula of alkynes is ${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{\mathrm{n}}$

So the ratio of carbon atoms to hydrogen atoms is $1:2$