Time taken to fill half of the cistern = 18/2 = 9 hrs
After half the cistern is filled , three more similar tubes are opened.
We know that one tube can in 18 hrs can fill = V litres(i.e. fully fill a cistern)
so four tubes in 18 hrs can fill = 4*V
18 hrs => 4*V litres - eq(1)
Since we need to fill only half of the cistern(i.e. V/2), we divide eq(1) by 8
18/8 hrs = V/2 litres
(18/8 *60 min = 135 min i.e. 2 hrs 15 min)
therefore time taken to fill remaining V/2 will be 2 hrs 15 min
Total time = 9 hrs + 2 hrs + 15 min = 11 hrs 15 min