You have the equations set up correctly. I will use r and b as the variables to make it easier to follow.
12r + 8b = 70
20r + 12b = 110
First let's solve one of the equations for one of the variables.
12r = 70 - 8b
[tex]r = \frac{70-8b}{12} [/tex]
Now plug that in for r in the second equation.
[tex]20( \frac{70-8b}{12})+12b = 110 [/tex]
Simplify
[tex]20(70-8b)+144b=1320[/tex]
[tex]1400-160b+144b=1320[/tex]
[tex]-16b = -80[/tex]
b = 5 grams
Now plug 5 in for b in one of the original equations to find r.
12r + 8(5) = 70
12r = 30
r = 2.5 grams
So the red marbles weigh 2.5 grams, and the blue marbles weigh 5 grams.
