A solid weighs 80g in air, 68g in water and 60g in oil. Calculate the relative density of oil and solid.

The relative density of an object can also be defined as its weight in air divided by the difference of its weight in air and its weight when the object is submerged in water. That is,
 $D=\frac{W_a}{W_a-W_w}$
On substituting the values in the above equation we get,
 $$\begin{gathered} D=\frac{80 g}{80 g-68 g} \\ D=\frac{80 g}{12 g} \\ D=6.67 \end{gathered}$$
So, as the relative density of solid is $D=6.67$.
Similarly, the relative density of oil can be calculated as,
 $$\begin{gathered} D\text{'}=\frac{loss in weight in oil}{loss in weight in water} \\ D\text{'}=\frac{W_a-W_{oil}}{W_a-W_{water}} \end{gathered}$$
On substituting the values we get,
 $$\begin{gathered} D\text{'}=\frac{80 g-60 g}{80 g-68 g} \\ D\text{'}=\frac{20 g}{12 g} \\ D\text{'}=1.67 \end{gathered}$$
Thus, the relative density of oil is $D\text{'}=1.67$