A housing complex built by DLF consists of 25 large buildings and 40 small buildings. Each large building has 15 floors with 4 apartments on each floor and each small building has 9 floors with 3 apartments on each floor. How many apartments are there in all?

Number of large buildings $= 25$
Number of floors $= 15$
Number of apartments on each floor $= 4$
∴ Total number of apartments in large buildings $= 25 \times 15 \times 4$
Number of small buildings $= 40$
Number of floors $= 9$
Number of apartments on each floor $= 3$
∴ Total number of apartments in small buildings $= 40 \times 9 \times 3$
Hence, the number of apartments in all $$\begin{gathered} = 25 \times 15 \times 4 + 40 \times 9 \times 3 \\ = 1500 + 1080 \\ = 2580. \end{gathered}$$