Surface Area of a Molecule

We can find the solid angles that common part subtends at the centre. We find that the common part subtends $\frac{\pi}{4}$ on the centre of Z and $\frac{5\pi}{8}$ on the centre of A. We know that the total solid angle is $4\pi$ , therefore the part on the outside for A and Z can be found out by subtracting from $4\pi$ .

Outside part of A = $4\pi - \frac{5\pi}{8}$ = $\frac{27\pi}{8}$ and Outside part of Z = $4\pi - 2\cdot\frac{\pi}{4}$ = $\frac{7\pi}{2}$

Therefore total outer surface area = $2 \cdot \frac{27\pi}{8} \cdot 40^{2} + \frac{7\pi}{2} \cdot 60^{2}=23400\pi$

P.S. To calculate the solid angle, you can draw two circles with radius 60 and 40 and distance between centres as 80, and find the angle that the arc subtends. Then use the formula of solid angle as $2\pi[1-\cos\frac{\theta}{2}]$ , where $\theta$ is the angle that the common arc subtends on the centre.