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Milla is 12 years younger THAN Louis.
In 18 years, she will be two thirds his age. What is Louis' current age?
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In other words, in 18 years, Luis will be 12 years older than Milla and she will be 2/3 of his age.
It means that the difference 12 years of their ages at that time will be 1/3 of the Luis' age.
Hence, in 18 years, Luis will be 36 years old, while Milla will be 36-12 = 24 years old.
So now, Luis is 36-18 = 18 years old. ANSWER.