A jar contains 24 marbles. Some of these are green and others are blue.

Total number of marbles = 24.
Let the number of blue marbles be x.
Then, the number of green marbles = 24 − x

$\therefore \mathrm{P}$ (getting a green marble) $=\frac{\text { Number of favourable outcomes }}{\text { Number of all possible outcomes }}$

$=\frac{24-x}{24}$

But, $P$ (getting a green marble) $=\frac{2}{3}$ (given)

$\therefore \frac{24-x}{24}=\frac{2}{3}$

$\Rightarrow 3(24-x)=48$

$\Rightarrow 72-3 x=48$

$\Rightarrow 3 x=72-48$

$\Rightarrow 3 x=24$

$\Rightarrow x=8$

Thus, the number of blue marbles in the jar is 8.