Help with a drug calculation problem

Hi everyone!
Well we're having another drug calculations exam next week and fortunately, we were given a practice test to help prepare.
I am having difficulty understanding one problem in particular:
Order: Heparin 7500 units IVPB in 500cc N/S to run @ 50 units per hour.
Set the IV pump at ____ cc/hr to deliver 50 units/hr.
Now, I do know that the correct answer is 3.33 cc/hr per the key that was provided, but I can not understand why this is. Can someone please be so kind as to explain? :trout:
Thanks so much in advance!
JanetLast edit by imanubee on Nov 11, '07 

Nov 10, '07hey there.
you want to do a proportion pretty much (well.. this is how i like to do it)
it's 7500 units/500cc
sooo.. since you want 50units/hr
7500units 50units
500cc x
>cross multiply to get:
7500x=25000
Divide each side by 7500
You get 3.33
Make sense? 
Nov 10, '07order: heparin 7500 units ivpb in 500cc n/s to run @ 50 units per hour.
set the iv pump at ____ cc/hr to deliver 50 units/hr.
dose on hand: 7500 units in 500cc of normal saline
you want to get the dose to give expressed as cc/hour.
50 units/1 hour (dose desired) x 500cc/7500 units (dose on hand) = 3.33 cc/hour (dose to give). however, you would actually set the pump at 3cc/hour since most pumps, except some of the very new and sophisticated ones, can only be programmed with whole numbers. 
Nov 11, '07Ahathat makes sense! I could plug in the numbers to get the answer as was provided on the key, but just needed it explained for clarity. Thanks!!!