Explanation
Let's have R be Najma's age.
R
And we have a younger sister who is 6 years younger than Najma:
R−6
In 10 years, the sum of their ages will be 50. So in 10 years Najma will be:
R+10
and her sister will be
R−6+10
and the two added together will be 50:
(R+10)+(R−6+10)=50
How old are they now?
(R+10)+(R−6+10)=50
2R+14=50
2R=36
R=18
and her sister is
R−6=18−6=12