Posted by patel on .
a tank can be filled by one pipe in 3
.5 hours and emptied by another in 4.2 hours.if both ipes are running, how long will it take to fill an empty tank?

math 
Steve,
in work problems like this, think of the amount of work each person can do in the unit of time.
if pipe a can fill the tank in 3.5 hours, then in one hour it can fill 1/3.5 of the tank.
The other pipe b can fill 1/4.2 of the tank in one hour.
So, in one hour, both pipes working together can fill up
1/3.5 + 1/4.2 = 0.5238 of the tank
So, to fill the whole tank, it will take 1/.5328 hours, or 1.90909 = 1 10/11 hours
In general, if various workers can do the job in a,b,c,... units of time, then working together they can do it in n units of time:
1/n = 1/a + 1/b + 1/c + ... 
math 
Steve,
I see I misread the problem. It was answered correctly later on