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 × 15 × 4
Number of small building = 40
Number of floors = 9
Number of apartments on each floor = 3
∴ Total number of apartments in small buildings = 40 × 9 × 3
Hence, the number of apartments in all = 25 × 15 × 4 + 40 × 9 × 3 = 1500 + 1080 = 2580.