There are two common oxides of sulphur, one of which contains 50% ${\mathrm{O}}_2$ by weight, the other almost exactly 60%. The weights of sulphur which combine with 1 g of ${\mathrm{O}}_2$ (fixed) are in the ratio of

                 First oxide              Second oxide

Sulphur       50%                        40%

Oxygen        50%                         60%

In first oxide , 1 parts oxygen combines with sulphur = $\frac{50}{50}=1$

Similarly for second oxide = $\frac{40}{60}=0.67$

So the ratio is 1 : 0.67 or 3 : 2