A sail has an area of 4000 m² and a total mass of 18 kg…

Step 1. The intensity there. $I = \dfrac{1361\ \text{W/m}^2}{(0.8)^2} = 2126.6\ \text{W/m}^2$.

Step 2. The pressure. $p = \dfrac{(1 + \rho)I}{c} = \dfrac{(1 + 0.85) \times 2126.6\ \text{W/m}^2}{3.00 \times 10^{8}\ \text{m/s}} = 13.1\ \mu\text{Pa}$.

Step 3. The force. $F = pA = 13.1 \times 10^{-6}\ \text{Pa} \times 4000\ \text{m}^2 = 52.5\ \text{mN}$.

Step 4. The acceleration. We want $a$, so $a = \dfrac{F}{m} = \dfrac{0.0525\ \text{N}}{18\ \text{kg}} = 2.91\ \text{mm/s}^2$.

Step 5. The speed. The time is $20 \times 86\,400\ \text{s} = 1.728 \times 10^{6}\ \text{s}$. So $v = at = 5.04\ \text{km/s}$.