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Pharmacy Pre reg calculation help

You are required to extemporaneously prepare 20% w/w coal tar ointment. You have in stock 5% coal tar ointment. How many grams of coal tar extract should be added to 200g of stock cream to produce 20%w/w cola tar ointment.
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Original post by the pro
You are required to extemporaneously prepare 20% w/w coal tar ointment. You have in stock 5% coal tar ointment. How many grams of coal tar extract should be added to 200g of stock cream to produce 20%w/w cola tar ointment.


Can't work this out at all, please any help!
I think is 37.5g. as you have 200g and you need to add Xg to both the nominator and denominator to make it 20%
0.2=10+x/200+x, solve the equation and it will come as 37.5

The fact that your stock was 5% means it had 10g in 200g stock of cream already.
Hope it helps

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