The amount of such a substance in the urine per unit of time must have been provided by filtering exactly the number of milliliters of plasma that contained this amount. Therefore, if the substance is designated by the letter X, the GFR is equal to the concentration of X in urine (UX) times the urine flow per unit of time (V*) divided by the aerial plasma level of X (PX), or UXV*/PX. This value is called the clearance of X (CX). PX is, of course, the same in all pas of the aerial circulation, and if X is not metabolized to any extent in the tissues, the level of X in peripheral venous plasma can be substituted for the aerial plasma level. C= 2*100/10 = 20ml/min Ref: Ganong's review of medical physiology,23rd edition, page: 645