A rectangular frame 0.30 m long and 0.20 m wide is placed in a uniform electric field of 800 N/C…

Step 1. The angle with the normal. $\theta = 90^\circ - 25^\circ = 65^\circ$.

Step 2. The flux. $\Phi = EA\cos\theta = 800\ \text{N/C} \times 0.060\ \text{m}^2 \times \cos 65^\circ = 48 \times 0.4226 = 20.3\ \text{N m}^2/\text{C}$.

Step 3. The largest flux. It happens when the frame faces the field. $\Phi_{\max} = EA = 800 \times 0.060 = 48\ \text{N m}^2/\text{C}$.