The following steps need to be followed in order to solve this query:
Step 1) Unpolarised light is incident on P1. So, after coming out its intensity is reduced by half. So, I1 = I0/2.
Step 2) Note that the angle between first and second polaroid is 90º and the third Polaroid is introduced between the two. So, take its angle with the first Polaroid and find its angle with the second Polaroid. You will get 45º.
Step 3) Now, find the intensity of light between the first and the third Polaroid. Use the relation I3 = I1 cos245. You should get I3 = I0/2 × 1/2 = I0/4
Step 4) Now, use this value of I3 to find the final intensity from the second polaroid. Use I2 = I3 cos245. You should get I2 = I0/4 × 1/2 = I0/8.