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

When she's 22 he'll be 11.

Mary is 16yr old. Her brother is obviously 4yr old. And x years from now Mary will be twice as old as her brother. Keep in mind that both of them will have to age the same number of years. Thus:

(16 + x) / 2 = 4 + x
Rearrange
16 + x = 8 + 2x
x = 8
So, in 8 years, when Mary will be 24yr and her nameless brother 12yr respectively, she just so happens to be twice as old as he is.

Mary will be 24 years old,her bro 'll b 12 years old.
Explanation:
Let the age of Mary when she's twice her bro be X
therefore,her brother's age will be 16/4+(X-16)
A.C:--

[4+(X-16)]*2=X
2*(X-12)=X
X=24

12