17%
Any object that is floating will display the volume of fluid that matches the mass of the object that's floating. So if the object is 100% as dense as the fluid, it will just barely sink, if it's 50% as dense as the fluid, 50% will be submerged, etc. So let's see what percent of mercury's density is concrete.
2.3x10^3 / 13.6x10^3 = 0.169117647 = 16.9117647%
Rounding to 2 significant figures gives 17%.
So 17% of a piece of concrete will be submerged in mercury when it's floating in mercury.
