Total volume dug by 600 men in 30 minutes = 5.5 x 4 x 405 cubic m
Total volume dug by 600 men in 1 hour = 2 x 5.5 x 4 x 405 cubic m
Total volume dug by 600 men in 6 hour = 6 x 2 x 5.5 x 4 x 405 cubic m
Total volume dug by 100 men in 6 hour = 2 x 5.5 x 4 x 405 cubic m
Total volume dug by 2500 men in 6 hour = 25 x 2 x 5.5 x 4 x 405 cubic m

Given that 2500 men dig in 6 hours = 10 x 8 x Length cubic m
=> 10 x 8 x Length = 25 x 2 x 5.5 x 4 x 405
=> Length = 25 x 5.5 x 405 / 10 = 5 x 55 x 405 / 20
=> Length = 5568.75 m

Option B