Question

A pharmacist needs 500 milliliters of a 20% phenobarbital solution but has only 5% and 25% phenobarbital solutions available. Find how many milliliters of each she should mix to get the desired solution.

Answer 1

Let x be ml of 5% soln.

let 500-x be ml of 25% soln.

.

x*.05=.05x

(500-x)*.25=125-.25x

500*.20=100

.

.05x+125-.25x=100

-.2x=-25

x=-25/-.2

x=125 ml of 5% soln.

500-x=375 ml of 25% soln.

.

Check:

125*.05= 6.25

375*.25=93.75

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