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Compounding Quantity Needed

by Amber

A custom preparation batch calls for a several liters of 8% alcohol solution. The pharmacy manager has asked you to check and calculate how much 8% can be compounded with stock on hand. You only find 1500mL bottle of 20% alcohol solution. What quantity will you report?

A. 4250 mL
B. 4500 mL
C. 3750 mL
D. 2250 mL

Comments for Compounding Quantity Needed

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Use Algebra on this one
by: Jerry

1) What are your factors:

What you have - - = - - What you want
(1500mL of 20%) - = - - (How many mL of 8%)

2) This is an equation calculation:

(1500mL)(20%) = (X mL)(8%)

3) Simplify the terms and do the Algebra:

(1500)(0.20) = (X)(0.08)

4) Keep breaking down to isolate X

300 = 0.08X

5) Solve for X

300 - - 0.08X
---- = -------
0.08 - - 0.08

6) X = 3750

- An alligation could be used on this one also, but this Algebra is quicker and cleaner for this scenario.

Pharmacy Tech for 4 yrs.
by: Kristina M.

I really like your explanation a lot sir. This is make everybody understanding very well. Thank you very much for showing the math.

by: Anonymous

the best explanation for this question. simple and straight to the point.

Compounding Quantity
by: Anonymous

Easier way is to use ratios. 20% is 2.5times greater than 8% (just do 20/8) and then multiply the total volume (1500) by 2.5 = 3750ml
EXL 20/8= 2.5 x 1500 = 3750ml

very nice explanaions
by: Anonymous

I really like both explanations, but the second one is the easiest one for me. Thank you very much for explaining

by: Anonymous

20%/8% *1500=3750

Alligations vs. V1 x C1 = V2 x C2
by: Anonymous

I was taught to use alligations to solve problems like this, how come if you use alligations you don't come up with the same answer?

Doing with Alligation
by: Jerry

Attn: Anonymous

If you get a different calculation using alligation, it means you're doing the alligation wrong.

20% | - | 8 Parts
- - | 8%| - - -
0% | - | 12 Parts

------- = 20 Parts total

And so,
- If 8 of the parts = 1500mL, then what volume is 20 parts?

1500mL - - X -
------ = -------
8 part - 20 part

Does this help? Maybe your syntax was off a little?

by: Anonymous

Thank you very much for the amazing explanation really kind of hard question... appreciate it

by: Anonymous

its easy to understand the explanation. thank u for showing how to get the answer.

by: Joni

For some reason, I get stumped on these questions. I got the right answer in a round-a-bout way but explanation one was much more concise.

thank you!
by: manda

thank you! that was very helpful!

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