A conducting sphere of radius R and carrying charge Q is joined to an uncharged conducting sphere of radius 2R. The charge flowing between them would be

$\frac{2Q}{3}$

$\text{Charges will flow such that final potential of the two spheres become equal} $

$\text{Let q charge flow from smaller sphere to large sphere}$

$V_1 = V_2$ 

$\Rightarrow \frac{k(Q -q)}{R} = \frac{kq}{2R}$

$\Rightarrow 2Q - 2q = q $

$\Rightarrow q = \frac{2Q}{3}$