5.3 minutes. First, let's calculate the volume of the tank. That will be the area of the base multiplied by it's length. And since it's base is a circle, the area is pi*r^2. So: V = l*pi*r^2 V = 3.5 * pi * (2.2/2)^2 V = 3.5 * pi * (1.1)^2 V = 3.5 * pi * 1.21 V = 4.235 * pi V = 13.30464489
So the tank has a volume of about 13.3 ft^3. Now simply divide that volume by the rate of incoming fluid. So 13.3 / 2.5 = 5.32 Rounding to 2 significant figures gives a time of 5.3 minutes.