Calculate the mass of a gas with a density of 0.0065 g/cm3 and volume of 260 cm3.

Mass=Density*Volume 
260 * 0.0065=1.69
Final
1.69g

There are numerous information's already given in the question. Based on these information's the answer to the question can be easily determined.
Density of the gas = 0.0065 g/cm^3
Volume of the gas = 260 cm^3
Then
Mass of the gas = Volume * density
                          = 0.0065 * 260 g
                          = 1.69 g
From the above deduction, we can conclude that the mass of the gas is 1.69 g. I hope the answer helps you.

The density of water is 1 g/cm cubed and in order for the pebble to sink its density has to be greater than the density of water and in order for the twig to float its density has to be less than the density of water