Question 428523
Emptying the tank is one job						
one pipe drains in	5	days				
It does 	  1/ 5	of the job in 1 	day 			
other pipe	7	day 				
It does 	  1/ 7	of thejob in 1 	day 			
Together they will do  1/5+  1/ 7		
Together they will do  12/35 of the job in one day 
So they will take 2 8/9	days			
So they will take 2.92	days