A venturi meter measures the flow of water in a 75mm diameter pipe. The difference of head between the throat and the entrance of the meter is measured by a U tube containing mercury, the mercury being in contact with the water. What should be the diameter of the throat of the meter in order that the difference in the level of the mercury to be 250 mm when the quantity of water flowing in the pipe is 620 dm3/min? Assume the coefficient of discharge of the meter to be 0.97.