The first pipe operates at rate of (6600 L / 66 min) = 100 L/min.
The second pipe operates at a rate of (6600 L / 55 min) = 120 L/min.
Working together, both pipes operate at a total rate of 220 L/min.
Then the time it takes for both of them to fill the tank is
(6600 L) * (1/220 min/L) = 30 min