maths
posted by patel .
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 ?

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) 
I also misread the problem. It is being emptied at 4.2 hrs.