Unpolarised light of intensity 200 W/m² falls on three Polaroid sheets in a row…

Step 1. The first sheet. $I_1 = \dfrac{I_0}{2} = \dfrac{200\ \text{W/m}^2}{2} = 100\ \text{W/m}^2$.

Step 2. The second sheet. The angle between the first and second sheets is $45^\circ$. $I_2 = I_1\cos^2 45^\circ = 100\ \text{W/m}^2 \times 0.50 = 50\ \text{W/m}^2$.

Step 3. The third sheet. The angle between the second and the third is $90^\circ - 45^\circ = 45^\circ$. $I_3 = I_2\cos^2 45^\circ = 50\ \text{W/m}^2 \times 0.50 = 25.0\ \text{W/m}^2$.