Here are some densities: water 1000 kg/m³, petrol 740 kg/m³, cork 240 kg/m³, teak wood 650 kg/m³…

Step 1. (a) On water, $1000\ \text{kg/m}^3$. Cork ($240\ \text{kg/m}^3$) and teak ($650\ \text{kg/m}^3$) are less dense, so they float. Rubber ($1150\ \text{kg/m}^3$), glass ($2500\ \text{kg/m}^3$) and gold ($19\,300\ \text{kg/m}^3$) are denser, so they sink.

Step 2. (b) On petrol, $740\ \text{kg/m}^3$. Cork ($240\ \text{kg/m}^3$) and teak ($650\ \text{kg/m}^3$) are still less dense, so both float on petrol too. The others sink.