Answer: 4 ounces per drop
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Step-by-step explanation:
"Per drop" means "per 1 drop" or "for each 1 drop".
We start off with the ratio of
3 drops: 12 ounces
Divide both parts of that ratio by 3 so that "3 drops" turns into "1 drop"
Doing so gets us the new ratio of
1 drop: 4 ounces
So he uses 4 ounces per drop.