A dehydrated patient needs a 7.9% saline IV. Unfortunately, the hospital only has bags of 3% and 10% saline solutions. How many liters of each of these solutions should be mixed together to yield 3 liters of the desired concentration?

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I pasted the answer here for you.

x = liters of 3% solution
3 - x = liters of 10% solution
.03x = value 3% solution
.10(3 - x) = value 10% solution
3 = liters of 7.9% mixture
.079(3) = 0.237 = value 7.9% mixture

.03x + .10(3 - x) = 0.237

Solve for x, which is number of liters of 3% solution.
(3 - x) = number of liters of 10% solution