From 6 different novels and 3 different dictionaries, 4 novels and 1 dictionary are to be selected and arranged in a row on a shelf so that the dictionary is always in the middle. Then the number of such arrangements is

less than 500

at least 500 but less than 750

at least 750 but less than 1000

at least 1000

Answer

at least 1000

N₁N₂N₃ – N₆, D₁D₂D₃
The number of ways = 6c₄ × 3c₁ × 24
= 15 × 3 × 24 = 1080