For these problems you do the ordered dose in the numerator then you multiply it by the form available (i.e., tablets in this case)/dosage.
I always put the dose ordered first, it always goes in the numerator. Then, you write the rest of the equation to set up to cancel out unwanted units. *REMEMBER*
The cancelations of units of measure are done either diagonally (cross) or vertical – NEVER
horizontal – so, in both of your problems, since mg is the unit in numerator of the ordered dose, you need mg to go in the denominator in fraction of the what's on hand. Another thing our instructor taught us was that when you look at the dose on hand (in this case 0.25 mg/tablet), you put what matches the unit of dose ordered in the denominator (in this case, 0.25 mg) and you ALWAYS put what units you want in the numerator (in this case, you want tablets, so that goes in the numerator)
Then, you cancel out the units (mgs) and you multiply across the numerators and across the denominators, then you divide the numerator and denominator, and you come up with the wanted quantity (in this case, the number of tablets to give your patient).
So, you give your patient a 0.5 mg tablet
- 0.125 mg goes first - in the numerator
- 0.25 mg/tablet goes next
- tablets in the numerator (because remember, you put what you want to know in the numerator - and you want to know how many tablets to give the patient)
- 0.25 mg goes in the denominator, because you want to match up the units you can cancel diagnonally - in this case equation starts with mg in numerator, so you want this fraction to have mg in the denominator so they can cancel each other out)
- Cancel out like units - cancel out both mg's.
- Multiply across the numerators - 0.125 x tablets = 0.125. Then multiply across the denominators - 0.25 mg
- Solve the problem - 0.125/0.25 = 0.5 tablets
You do the same for your second problem
= 150 mg
Dose on Hand
= 300 mg/tablets (form
= tablets, dosage
= 300 mg)
Put ordered dose in numerator of first fraction (I forgot to mention this before, you can put a 1 in the denominator, or you can just leave it blank (whatever makes you comfortable). Since this is only a one-factor problem, there is only something in the numerator. If you were doing a dosage calculation based on weight, for example, then you would have ordered dose in numerator and kg or lb in denominator)
Then second fraction is what you have on hand – form goes in numerator (in this case tablets) (on a side note, if you were trying to figure out mL to give, then dosage in mL would go in numerator (e.g., 15 mL)). Then the dosage on hand goes in denominator (in this case 300 mg) – remember you want to be able to cancel out the mg which can only happen diagonally, so it needs to go in the denominator to cancel out the mg in the numerator of the first fraction.
Then multiply across the numerators: 150 x tablets = 150
Multiply across the denominators: 300 = 300
Divide numerator and denominator: 150/300 = 0.5 tablets
You would give the patient 0.5 tablets
Hope this helped some!
By the way, this is called dimensional analysis. Our instructor had us try all different types of equations to solve dosage calculations to find which one we felt most comfortable with. For me it was dimensional analysis.
I used this book: https://web.opendrive.com/api/v1/dow...O6emK?inline=1
Some Good Sources
Uses Dimensional Analysis
: Dosage calculations the easy way! - Straight A Nursing
Sources that Show Different Methods of Calculations
(e.g., basic formula, ratio proportion, fractional method, dimensional analysis)
nursesaregreat.com - Homepage
Sources That Show Simple to Harder Dosage Calculation Problems:
The nurse's quick guide to I.V. drug calculations : Nursing made Incredibly Easy
DosageHelp.com - Helping Nursing Students Learn Dosage Calculations
Hope this helps you!