A dairy farmer wants to mix a 80% protein supplement in a standard 70% protein ration to make 1500 pounds of a high-grade 75% protein ration. How many pounds of each should he us

Answer 1

clearly, since 75 is just the average of 70 and 80, he should use 750 lbs of each.

Or, more generally, if he has x lbs of the 70% mix, then
70x + 80(1500-x) = 75*1500
solve and you will see that x = 750.
But this method works for less obvious mixtures.