Age Matters

Mary's brother is currently $\frac{16}{4} = 4$ years old. There are two nice ways to proceed:

1) The difference between their ages is $16-4=12.$ Thus, since the difference is fixed, we know that the ages where Mary will be twice as old as her brother must be $12$ and $12\times 2 = 24,$ so when Mary is 24.

2) If $n$ more years pass, Mary is $16+n$ while her brother is $4+n.$ Setting $16 +n = 2(4+n),$ we get $n=8,$ so Mary will be 24.

when she was 16, she was 4 times her brothers age, which means her brother was 4. She is 12 years older than him, and when she will be 24, it will be twice her brother's age.

Quicker way to figure this out? Just find the difference of ages which is 16, for Mary, and 4, for her brother. So the answer will be 12. Then, you multiply that by 2 to obtain 24 which is the answer.

If the brother's present age is x. Then 16 = 4x. As such the brother's present age will be 4. Suppose in y years time, Mary is twice as old as her brother. Then 16 + y = 2(4 + y), implying y = 8. This means that Mary will be 16 + 8 = 24 when she will be twice as old as the brother

Age of brother = x, Age of Mary 4x = 16: So 16/4= 4, Age of brother = 4. Differnce of age 16 -4 = 12, When age of Mary =24, age of brother= 12. ie Age of Mary is twice old as her brother.