A nurse wishes to to add water to 60 ounces of 20% solution of benzalkonium chloride to dilute it to an 8% solution. How many ounces of water does she need to add?

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420 mL

To solve this problem, we need to find out how many ounces of water the nurse needs to add to the 20% solution in order to achieve an 8% solution.

Here's the step-by-step process to get the answer:

Step 1: Set up the equation.
Let x represent the number of ounces of water the nurse needs to add.
The total volume of the diluted solution will be 60 ounces + x ounces.

Step 2: Write the equation based on the concentrations.
In the 20% solution, the amount of benzalkonium chloride is 20% of 60 ounces, which is 0.2 * 60 = 12 ounces.
In the diluted 8% solution, the amount of benzalkonium chloride is 8% of the total volume, which is 0.08 * (60 + x) ounces.

Step 3: Equate the two amounts of benzalkonium chloride.
Since the amount of benzalkonium chloride stays the same before and after dilution, we can set up an equation:
12 ounces = 0.08 * (60 + x) ounces.

Step 4: Solve the equation.
To solve for x, divide both sides of the equation by 0.08:
12 / 0.08 = 60 + x
150 = 60 + x

Next, subtract 60 from both sides of the equation:
150 - 60 = x
90 = x

So, the nurse needs to add 90 ounces of water to the 20% solution to dilute it to an 8% solution.