SOLUTION: I need help solving problems like A nurse wishes to add 30 ounces of a 10% solution of benzalkonium Chloride to dilute a 4% solution. How much water should she add?

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Question 907018: I need help solving problems like
A nurse wishes to add 30 ounces of a 10% solution of benzalkonium Chloride to dilute a 4% solution. How much water should she add?
Found 2 solutions by josgarithmetic, richwmiller:
Answer by josgarithmetic(39617) About Me  (Show Source):
You can put this solution on YOUR website!
Only a guess that the starting 30 is as fluid ounces and the salt is percent mass per volume. The nurse would add some v fluid ounces of water:


30%2A10%2F%28v%2B30%29=4
300=4%28v%2B30%29
4v%2B120=300
4v=180
v=%282%2A5%2A2%2A9%29%2F%284%29
highlight%28v=45%29 fluid ounces of water
(This checks as correct.)


CHECK ANSWER:
30%2A10%2F%2845%2B30%29
300%2F75
simplify
25%2A12%2F%2825%2A3%29
12%2F3
highlight%284%29
Check is perfect.

Answer by richwmiller(17219) About Me  (Show Source):
You can put this solution on YOUR website!
We have 30 oz of 10% solution
We want to add some 0% solution to make 4% solution
0*x+0.1*30=0.04(30+x)
0*x+3=1.2+0.04x
0x-0.04x=1.2-3
-0.04x=-1.8
x=45 oz of 0% (0) is added to 30 oz at 10%(3) to make 4% of 75 oz(3)
check
0*45+0.1*30=0.04(30+45)
0+3=0.04(75)
3=3
ok