Question 909155
let x = Nana's present age 
y = her sister's present age 

in five years time: 
x+5 = Nana's age 
y+5 = her sister's age
{{{y+5 = 3(x+5)}}}

fours years ago: 
x-4 = Nana's age
y-4= her sister's age
{{{y-4=6(x-4)}}}

Equations:
{{{y+5= 3(x+5)}}} = {{{y-3x-10=0}}}
{{{y-4=6(x-4)}}} = {{{y-6x+20}}}
Elimination: 
 {{{y-3x-10=0}}}+ {{{-y+6x-20}}}= {{{3x-30=0}}}
{{{x=10}}}
{{{y+5=3(10+5)}}}
{{{y+5=30+15}}}
{{{y=40}}}

Nana: 10 years old Her sister: 40 years old