#4) 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?

Hi! Tricky question you've got there!

So far, we know that Mary is 16 and her brother is 4 (16 is 4 times as old as 4).

16 + x = 2(4 + x)

16 + x = 8 + 2x

x = 8

Now, this isn't finished yet.

(4 + 8) = 12

12 twice is 24, so Mary will be 24 years old.