Blood levels and Plasma levels...

chound

In my physiology examination, I came across this question:
Find the Glomerular Filtration Rate (GFR), when
Urine Concentration of Urea (U) = 70 (I dont remember the exact values, but it is of no importance),
Urine flow rate (V) = 1ml/min
Blood level of Urea = 20 ( or something)
In Guyton(Physiology text book), The formula was given as GFR = U*V/P
P is Plasma level of Urea.
My doubt is whether I should convert the Blood Conc into Plasma Conc ( as Blood is 5 L while Plasma is 3 L) and then continue with the problem?