Explanation:

Let the efficiency of 1 man be 'E',
Total work done by N men = N x E x 15 = 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
The required value of N is 15.