Question 489598
Emptying the pool is one job		1 job					
Pipe I  	20	minutes					
She does	  1/20	job in 1	hour				
Pipe II 	30	minutes					  1/12
He does 	  1/30	of thejob in 1 	hour				
Pipe III	40	minutes					
It does 	  1/40	of the job in 1 	hour				
							
Together they will do 		  1/20	+	  1/30	+	  1/40	
Together they will do 		  9/83		of the job in one 		minute	0.11
So they will take 		9 2/9	minutes				
So they will take 		9.23	minutes