As given here, the pass axis of P2 and P3 are inclined at angle of 60º and 90º with respect to axis of P1.
Therefore, intensity of light after falling on first polaroid P1,I1=2I0
Intensity of light after falling on second polaroid P2,I2=I1cos260∘=8I0
Intensity of light after falling on third polaroid P3,I3=I2cos230∘=8I0(43)=323I0
The intensity of light at O is 323I0=323×256=24Wm−2
