ln the figure shown neglecting friction and mass of pulleys, what is the acceleration of mass B?

For block A :

mg - 2T = ma---------------(1)

For block B;

T-mg = mb----------------(2)

And by constraint motion,

(a-b)+a = 0--------------(3)

Solving (1), (2) and (3) get

a = $-\frac{\mathrm{g}}{5}$

and b = $\frac{-2\mathrm{g}}{5}$