Question 746929
Lets say Mary is 20 years old. Then 7 years ago she would have been 13 years old 
that is  
{{{ 20 - 7 = 13 }}}

In 10 years time Mary will be 30, that is:
{{{20 + 10 = 30 }}}

So lets do the same thing but instead of saying Mary is 20 years old, we will say Mary is n years old.
Then 7 years ago Mary would have been {{{n - 7}}} years old.

In 10 years time Mary will be  {{{n + 10}}} years old!

Simplez