let the present age of her brother be x.

then her present age is 4x.

also given that 4x = 16.

so x = 4.

present age of her brother is 4.

the difference between their ages is 12 and it remains constant...

the age after y years of her brother will be 4+y.

then her age will be 16+y.

16+y = 2(4+y) = 8 + 2y.

so we have y  =  8 years.

after 8 years her age will be 24.

her brother's age will be 4 + 8 = 12 years..

 Sorry , the answer wasnt punctuated . 

We have ,

present age of Mary = 16

hence her brothers age = 4

now difference in their age = 12

When Mary is aged double her brother( be x age ) would be 2x . But the difference in their age stays

.therefore ,

2x - x = 12

so x = 12

Brothers age = 12

marys age =24