The total number of cups is 8.
We know that 20% is wild birdseed, this means:
[tex]8 \cdot \frac{20}{100} = 1.6[/tex] cups of wild birdseed
and
[tex]8 \cdot \frac{80}{100} = 6.4[/tex] cups of sunflower seeds
In total she will put 1.6 + 6.4 = 8 cups of birdseed mix