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
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.