before getting into the solution to your question lets just get some background:

To put it in simple terms, specific gravity is the factor by which something is denser than water.

If there is an oil that has a mass of 2 KG/1000 ml then as compared to water (which is approx. 1kg/1000 ml) the oil is twice as dense and therefore the oil has an S.G. of 2.

Now, in your question, just equate the weight of the liquids displaced (this is because the weight of each liquid displaced will be the same as the weight of the object). An object floats in a solution when it displaces an amount of the liquid that is equal to its weight.

Let the weight of the object be Wo.

Let the total volume of the object be Vo.

The density of the object be Do

Density of Water is Dw and that of CCl4 is Dc.

So in the case of the object floating on water, volume of water displaced is 0.75*Vo

So, Wo = 0.75*Vo*Dw.................1

Similarly, In the case of CCl4, Volume displaced = 0.5 * Vc

again Wo = 0.5*Vo*Dc..................2

Equate 1 and 2

So, 0.75*Vo*Dw = 0.5*Vo*Dc

Specific gravity of CCl4 is Dc/Dw = 0.75*Vo/0.5*Vo

Dc/Dw = 0.75/0.5 = 1.5

Sorry if I've made it look complicated...