Posted by patel on Thursday, October 20, 2011 at 6:05pm.
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 + ...
I see I misread the problem. It was answered correctly later on
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