atomic mass of carbon is C = 12 g/mol
atomic mass of hydrogen is H = 1 g/mol
now number of carbon atoms
[tex]N_c = \frac{120}{12} = 10[/tex]
Similarly the number of hydrogen atoms are
[tex]N_H = \frac{30.3}{1} = 30[/tex]
so Carbon atom and hydrogen atom must be in ratio of 10:30
so the ratio is 1:3
now the empirical formula is always in simplest form
[tex]CH_3[/tex]