Question 378447
Let k stand for Karen's age, and m stand for Mary's age.

2m-8=k
k+m=28


Substitute k for k=2m-8
(2m-8)+m=28
3m-8=28
3m=36
m=12


Substitute m for m=12
k+m=28
k+(12)=28
k=16


Karen is 16 years old, Mary is 12 years old.