Pipe A alone takes 5 hours to fill a tank thus the rate of pipe A is: 1/5 tank per hour

let 'x' represent the number of hours that pipe B needs to fill the tank, x>0

Pipe B has a rate of: 1/x tank per hour

When pipes A and B are used simultaneously, it takes 2 hours to fill a tank thus the together rate is: 1/2 tank per hour

1/5 + 1/x = 1/2

by solving we find:

x = 10/3 hr

x = 3 hr 1*60/3 min

x = 3 hr 20 min

click here to see the step by step solution of the equation:

Pipe B alone takes 3 hours and 20 minutes to fill a tank.