Question 578599
Filling	the	pool	is one job
Pipe I	18	hours	
It does	  1/18	of the job in 1	hour
Pipe II	22	hours	
It does	  1/22	of thejob in 1	hour 

together they will do 1/18  + 1/22 of the job in 1 hour

Take the LCD

(22+18)/22*18

40/396

=10/99 of the job they complete in 1 hour

so they will take 99/10 hours

= 9.9 hours

OR 9 hours & 54 minutes  ( 0.9 *60 = 54)