We will need 85cc of 20% saline solution for the mixture
Let the amount of 20% saline solution be "a"
If a nurse mixes 40 cc of a 70% saline solution with a 20% saline solution to produce a 36% saline solution, then the finished product will be 40 + a cc which containing 36% saline solution.
The saline content will therefore be expressed as:
(70% of 40)+20% of a = 36% of (40 + a)
0.7*40 + 0.2a = 0.36(40 +a)
28 + 0.2a = 14.4+0.36a
28 - 14.4 = 0.36a - 0.2a
13.6= 0.16a
a = 13.6/0.16
a = 85
This means that we will need 85cc of 20% saline solution for the mixture
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