How long for 2 pipes working together to fill a tank?

[Shinbacci]

Pipe A can fill a tank in 3 hours. Pipe B can fill the same tank in 4 hours. If both pipes are on, how long will it take the tank to fill?

 

[MarkFL]

Hello, and welcome to FMH!

Working together, how many tanks could the two pipes fill in 12 hours?

 

[MarkFL]

To follow up, in 12 hours pipe A can fill 4 tanks, and pipe B can fill 3, so working together, they can fill 7 tanks in 12 hours, thus they can fill 1 tank in 12/7 hours, or about 1 hour 42 minutes 51.43 seconds.

 

[Denis]

Pipe A can fill a tank in 3 hours. Pipe B can fill the same tank in 4 hours.
If both pipes are on, how long will it take the tank to fill?

You can "see" what's agoin' on by making the "job" = travelling a distance.
As example, make the "job" travelling 120 miles:
A@40 mph..................120 m..................>3 hr
B@30 mph..................120 m..................>4 hr
A+B@70 mph.........................................................840 m.........................................................>12 hr

Playing around with that will lead you to Mark's answer.