Inulin was infused into antecubital vein of a healthy human subject whose GFR is 125ml/min. The inulin was infused at a steady rate of 75mg/min and periodically blood was withdrawn from the determination of the plasma inulin concentration, Pin. Pin reached a steady state value of 0.6mg/ml after 120 min. Calculate the urinary excretion of inulin in mg/min at this time. Show the calculation procedure that you used and explain the validity of using inulin in this manner