Two taps A and B fill a tank in 6 minutes, tap A fills tank in 10 minutes, how long does tap B take to fill the tank?

Are you asking because you do not know or because you want to see if we know?

Tap A fills the tank in 10 minutes, so it's flow rate is 1/10 tank per minute. Both taps together fill the tank in 6 minutes, so they have a combined flow rate of 1/6 tank per minute.

flow rate A + flow rate B = combined flow rate

1/10 + B = 1/6

Now just solve for B, which will be the flow rate from tap B (in tanks per minute). Then just take its reciprocal to get the time it takes to fill the tank (in minutes per tank).