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

Sure, let's solve this step by step:

1. First, we know that Mary is 16 years old and she is four times as old as her brother. So, her brother is 16 / 4 = 4 years old.

2. We need to find out when Mary will be twice as old as her brother. This means the difference in their ages will remain the same as they grow older. The difference in their ages is 16 - 4 = 12 years.

3. So, if we add this difference to the brother's age, we get 4 + 12 = 16 years. This means when her brother is 16 years old, Mary will be twice as old as him.

4. To find out how old Mary will be then, we add the number of years it takes for her brother to reach 16 to Mary's current age. So, 16 (brother's future age) - 4 (brother's current age) = 12 years. This is the number of years in the future when Mary will be twice as old as her brother.

5. Finally, we add this to Mary's current age: 16 (Mary's current age) + 12 (years in the future) = 28 years old.

So, Mary will be 28 years old when she is twice as old as her brother.