So here you're solving a system of equations. If A=the number of strokes Abby has, B=the number of strokes Brenda has, and C=the number of strokes Charda has, then your equations would be:
A-4=(A+B+C)/3
(A+B)+8=3C
A+B+C=192
If we look carefully, you can see that (A+B+C) can be seen in two equations (the first and third). So you could use substitution with that.
A-4=(192/3)
A-4=64
A=68
So now you know that Abby has 60 strokes, you can substitute that into any of the other equations. I chose the second and got:
68+B+8=3C
B+76=3C
B=3C-76
If you substitute this and Abby's score into the third equation, you get:
68+(3C-76)+C=192
4C-8=192
4C=200
C=50 strokes
