how many pounds of each should be mixed to make 80 lb of hamburger of hamburger that is 17% fat?

a butcher has some hamburger that is 20% fat and some hamburger that is 12% fat. 

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).