The percentage of C, H and N in an organic compound are 40%, 13.3% and 46.7% respectively then empirical formula is :

Divide each percentage by the atomic mass to get the relative number of moles:
C: $\dfrac{40}{12} = 3.33$; H: $\dfrac{13.3}{1} = 13.3$; N: $\dfrac{46.7}{14} = 3.33$
Divide by the smallest value (3.33): C = 1, H = 4, N = 1
So the empirical formula is $CH_4N$.
(The molecular formula could be a multiple, e.g. $C_2H_8N_2$, ethylenediamine.)