Three solid metal balls with radii 3 cm, 4 cm and 5 cm are melted and made into one ball…

Step 1. The volume. $R^3 = 3^3 + 4^3 + 5^3 = 27 + 64 + 125 = 216$, so $R = 6\ \text{cm}$.

Step 2. Before. The three balls have area $4\pi(9 + 16 + 25)\ \text{cm}^2 = 200\pi\ \text{cm}^2$.

Step 3. After. The new ball has area $4\pi \times 36\ \text{cm}^2 = 144\pi\ \text{cm}^2$.

Step 4. The fall. $200\pi - 144\pi = 56\pi\ \text{cm}^2$, which is $175.9\ \text{cm}^2$.