Mary, who is sixteen years old, is four times as old as her brother. How old will Mary be when she she is twice as old as her brother?

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Answer 1 · 2016-12-02T08:09:20

$24$ $24$ years old

When Mary is $16$ $16$ , her brother is $4$ $4$ .

Let $x$ $x$ be the year when Mary will be twice as old as her brother.

$16 + x = 2 (4 + x)$ $16 + x = 2(4+x)$

$x = 8$ $x = 8$

Therefore, Mary will be

$16 + 8 = 24 years old$ $16 + 8 = "24 years old"$

while her brother will be

$4 + 8 = 12 years old$ $4+8 = "12 years old"$