Posted by patel on Friday, October 21, 2011 at 9:09am.
a tank can be filled by one pipe in 3.5 hours and emptied by another in 4.2 hours. if both pipes are running, how long will take to fill an empty tank ?

maths  bobpursley, Friday, October 21, 2011 at 9:26am
rate=amount/time
rate combined=sum of rates=1tank/3.5hr+1tank/4.2hrs
rate combined=(4.2+3.5)/(3.5*4.2) tanks/hr work this out.
time= 1tank/(above rate) 
maths  Steve, Friday, October 21, 2011 at 12:54pm
I also misread the problem. It is being emptied at 4.2 hrs.