Question 921595
You need a variable that will be a measure of the time when your mother will be 3 times older than your niece.
If {{{n}}} years from now your mom is 3 times your niece's age, at that time
your mom will be {{{70+n}}} years old;
your niece will be {{{10+n}}} years old,
and their ages will be related by
{{{3(10+n)=70+n}}} .
That is your equation.
 
{{{3(10+n)=70+n}}}--->{{{3*10+3n=70+n}}}--->{{{30+3n=70+n}}}--->{{{30+3n+30-n=70+n-30-n}}}--->{{{3n-n=70-30}}}--->{{{2n=40}}}--->{{{n=40/2}}}--->{{{n=20}}}
Your mother will be 3 times older than your niece 20 years from now.
At that point, your mother will be {{{70+20=90}}} ,
and your niece will be {{{10+20=30}}} .