Question 829863
Let M = Mary's age
Let B = her brother's age
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Here's what we're told in the question:
16 = 4B
Divide each side by 4
4 = B (brother's age)
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The question is asking what M will be when M=2B.
Since B is 4, M is 4*2 or 8.
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Odd question because it reads "how old WILL Mary be..."  That time has already passed.