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 ?

Answer 1

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)

Answer 2

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