Proportions math is very commonly used by pharmacy technicians. It is mandatory to learn this process to prepare for the PTCB or ExCPT. A majority of the calculations you'll do use proportions math.

You must know 3 factors and be looking for the fourth (unknown or X).

Let's jump right into a practice question and see how this works.

Practice Question #1 : If Fifty tabs of Clarithromycin 500mg costs $25.00, how much would ten tabs of Clarithromycin 500mg cost ?

1) In this case, we know that 50 tabs cost $25.00, those are the first two factors. We also know we want to figure out how much only 10 tabs would cost, 10 is the third factor and its cost is the unknown.

This is how you want to write your two fractions:

$25.00

?

50 tabs

10 tabs

2) Next, we want to multiply the denominator from the unknown side with the numerator on known side and write the product to the left:

250.00 =

$25.00

←

?

X

50 tabs

←

10 tabs

3) Next, we want to take this product and divide it by the known sides denominator:

250.00

$25.00

→

= 5

÷

↑

→ →

50 tabs

→

10 tabs

4) The calculation you get goes into the numerator on the unknown side and that's YOUR ANSWER !

$25.00

$5.00

50 tabs

10 tabs

The next one is slightly harder.

Practice Question #2 : You are compounding a batch that requires 227g of drug F, which costs your pharmacy $94.00 per pound. How much does this ingredient cost the pharmacy for this batch.

1) In this case, we know that 1 Lb. cost $94.00, those are the first two factors. We also know we want to figure out how much only 227g would cost, which is the third factor and it's cost is the unknown.

This is how you want to write your two fractions:

$94.00

?

1 Lb.

227 g

2) In this case, The first thing you want to do is make all of your terms the same. You'll do this by converting 1 pound to 454 grams:

$94.00

→

$94.00

?

1 Lb.

→

454g

227g

3) Next, we want to multiply the denominator from the unknown side with the numerator on known side and write the product to the left:

21,338 =

$94.00

←

?

X

454 g

←

227g

4) Next, we want to take this product and divide it by the known sides denominator:

21,338

$94.00

→

= 47

÷

↑

→ →

454g

→

227g

5) The calculation you get goes into the numerator on the unknown side and that's YOUR ANSWER!

$47.00

Proportion math can be used for many calculations. Keep in mind that you need to always be dealing with like terms, want to have at least 3 known factors and be looking for the (fourth) unknown factor. If you are preparing for the Pharmacy Tech test and doing a lot of
Practice Quiz Questions
, you should try to use proportion calculations as much as possible. Write out all of the steps and keep organized. This will come as second nature to you sooner or later.

Question from Debra in Texas : How many 100mg tablets will you need to make 1/2 liter of a 1:250 solution?

1) First, use proportions math to calculate how many grams you need in 500ml (1/2 Liter).

1 gram

X

=

250mL

500mL

2) So, hopefully you got 2 grams for X. Now, the question then becomes, how many 100mg tablets would equal 2 grams. In order to use similar terms, let's write 2 grams as 2000mg.