Ok you're right that would be mass.

And I thought I was pedantic!   

Bob

hi Agnishom,

When an object floats in any liquid, the weight of liquid displaced = the upthrust on the object.

example.  ice is less dense than water.  The RD is about 0.91

An ice cube of 1 cubic centimetre is put in water.  It's weight = volume x RD = 0.91 g

So 0.91 g of water is displaced (pushed out of the way as the cube settles down in the water).  So 91% is below the waterline and 9% above.

But the RD is relative.  Because it depends what liquid you use as the reference that you measure other substances by.

If we lived on a liquid methane planet, we'd probably use that as the reference!

On this planet we usually use water as the reference and we have rigged the units so that 1 cc of water weighs 1 g; giving  water itself a density of 1.

Water will float on a heavier liquid, such as mercury.

Your example.

1/5 above the surface of the water so it's RD (= density = specific gravity)  is 0.8 

Let's say we have 1 cubic centimetre of the substance.  Then its weight is 0.8 g.

Now it is placed in another liquid and it floats with just 1/20 above the surface. 

That means it displaces 0.95 cc of that liquid and that amount of liquid must weigh 0.8 g (in order to hold the substance up).

So the density (specific gravity) of that substance (relative to water) must be = weight / volume = 0.8 / 0.95 = 0.84