Answer
15 rocks can be added before it sinks.
Work Step by Step
Since a third of it is under water, we know that the jar itself has a third of the density of water. Thus, we find how many rocks have to be added in the given volume to make the density of the rock 2/3 of that of the water. We call the number of rocks n to find:
$\frac{2g}{3cm^3} = \frac{12n}{\pi (2.5^2) (14)}$
$n = 15.27$
