There's a standard algebraic way to do this, but getting the reasoning will help you more in long run.
To reason this out, imagine that you wanted meat that was 16% fast. You'd put in 40 lb of each one, because the average of 20% and 12% is 16%. Since 16% is less than 17%, you can figure that you'll need more of the 20% than the 12%.
In the 80 pounds, you'll have X lb of 20% and Y lb of 12%. Because X and Y equal 80, you can use 80-X for Y. The equation is 20X + 12(80-X) = 17 (80).