let

m = 16 the present age of Marry

b = the present age of her brother

Mary who is sixteen years old, is four times as old as her brother

m = 4b

4b = m

4b = 16 divide both sides by 4

b = 4

lets say that after x years Marry is twice as old as her brother that is

16 + x = 2(4 + x)

by solving we find

x = 8 years

click here to see the step by step solution of the equation

after 8 years Marry will be 16 + 8 = 24 years old.