A work can be completed by N men in 15 days. 5 men leave the work after 3 days. The remaining work was completed in 18 days. What is the value of N?

15

Let the efficiency of 1 man be 'E'

Total work done by N men = N x E = 15NE

Total work done by N men in 3 days = 3NE

After 3 days 5 men left the work. Remaining workers = N - 5

The remaining work is completed in 18 days by (N - 5) workers,

⇒ 3NE + 18 x (N - 5) x E = 15 NE

⇒ 18(N - 5) = 12N

⇒ 18N - 90 = 12N

⇒ 6N = 90

⇒ N = 15.