Available digits: 1,2,4,5,7,8
Since the numbers had to be arranged in decreasing order.
Total number starting from 8 (8 _ _ _ _ _) will be 5!
Total number starting from 7 (7 _ _ _ _ _) will be 5!
Now the total numbers starting from 58 (58 _ _ _ _) will be 4!
Total numbers starting from 57 (57 _ _ _ _) will be 4!
Now the total numbers starting from 548 (548 _ _ _) will be 3!
The total numbers starting from 547 (547 _ _ _) will be 3!
The total numbers starting from 542 (542 _ _ _) will be 3!
Now the total numbers starting from 5418 (5418 _ _) will be 2!
Now the next number will be ′′541782′′
Total number before ′′541782′′ are 5!+5!+4!+4!+3!+3!+3!+2!=308
∴ The required position of ′′541782′′ will be 309