The first pump fills one tank in 2 hours, so it works at a unit rate of (1 tank)/(2 hours) = 0.5 tanks/hour.
The second pump takes 3 hours to fill the same tank, so it works at a rate of (1 tank)/(3 hours) ≈ 0.33 tanks/hour.
Working together, the pumps can fill one tank at a rate of
(1/2 + 1/3) tanks/hour = (3/6 + 2/6) tanks/hour = 5/6 tanks/hour ≈ 0.83 tanks/hour
Then the time it takes both pumps to fill one tank is
(1 tank) / (5/6 tanks/hour) = 6/5 hours = 1.2 hours = 72 minutes
