The intensity $I$ of the transmitted light varies directly as the square of the cosine of the angle between the transmission direction of polariser and the analyser.

$I=I_0{\cos }^2\theta$

Intensity of finally transmitted light after passing through the polaroid B is given by $I=I_0{\cos }^2\theta$ (Intensity)

$I=I_0\cdot {\cos }^2{30}^o$

$I=0.75I_0$

where, $I_0$ is the maximum intensity of transmitted light.