Oxide of Sulphur means the compound formed is bonding between Sulphur and Oxygen.

Also, if the compound contains 60% Oxygen, then it must contain 40% Sulphur

Elements  S  O

%by mass of atoms  40  60

Divide by their masses \(\frac{40}{32}\) \(\frac{60}{16}\) 

 1.25  3.75

Divide through by smaller value  \(\frac{1.25}{1.25}\) \(\frac{3.75}{1.25}\) 

 1  :  3

Empirical formula SO\(_3\)