Math Question

I've been practicing some math problems using the ratio/proportion method and got very confused about how to convert this particular problem. I would appreciate it if someone could help me out w/ this problem.
Mrs. Skinner was ordered to have gr(grain) 1/6 of morphine. You have available on hand morphine labeled 15 mg per ml. How many mls will you give your patient?
The answer that the book gave was 0.67 ml 

Aug 8, '03I think the first thing you need to know is the conversion from grains to grams, then from grams to mg then you can do the equation. Do you understand? Now i would give you the equivalent of grains to grams but then that would take the fun out of it.
When you have how many grains are in a mg, you should be able to do your ratio. I hope this is correct? Anyone else care to take a stab at it? 
Aug 8, '03here's a foolproof way to do all these types of problems.
1 gr (1 ml ) (.. 60 mg) 60
6.... (15 mg) ( 1 gr ) = 90 = 0.67 ml
first you cancel out gr's and then mg's. ml is left and that is what you want to be left so remember to always put your desired label in the numerator and set the rest of your problem up so that you can cancel out your undesired labels (gr and mg).
i added my conversion factor (60 mg = 1 gr) into my problem and then multiply accross.
in this case i multiplied 1 x 1 x 60 = 60
then in the denominators 6 x 15 x 1 = 90
then i divided, 60 divided by 90 = 0.67:d
this will work with all simple dosage calculations. i could be wrong so someone finding otherwise please feel free to correct me!
marci 
Aug 8, '03Thank you for the help guys! I guess my main problem was setting up that particular one. Good thing I'm getting a head start on this before I start nursing school in a couple of weeks.

Aug 8, '03Only method that we've learned to do this so far:
Amount Desired
 =Quantity
Amount on hand
D
=Q
H
gr 1=60 mg
so gr1/6=10 mg
10mg is what we want
 X 1 mL = 0.67 (rounded off)
15 mg is what we have 
