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Answer:
30 mL
Explanation:
The meaning of 60% solution is ...
alcohol/(total volume) = 0.60
You are told the amount of alcohol is 18 mL, so you have ...
(18 mL)/(total volume) = 0.60
Multiplying by total volume gives ...
18 mL = 0.60 × (total volume)
Then dividing by 0.60, you have ...
(18 mL)/0.60 = total volume = 30 mL
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Additional comment
You need to apply what you know about the real world to consider whether your answer makes any sense. If alcohol is a little more than half of the volume, and the amount of alcohol is 18 mL, then the total volume cannot be less than 18 mL. In fact, you expect it to be not quite double 18 mL.
An answer of 10.8 mL makes no sense in this context. You cannot have less total solution than you have of something that makes up a part of the total solution.