A person wishes to mix coffee worth $9 per lb with coffee worth $3 per lb to get 240 lb of a mixture worth $5 per lb. how many pounds of the $9 and the $3 coffees will be needed?

Let x = lbs of the 9$ mix

y = lbs of the 3$ mix

x+y=240

9x+3y=1200

Divide the second equation by - 3 then combine to get - 2x = - 160.

Divide - 160 by - 2 to get the number of lbs of the $9 mix, 80lbs.

Subtract 80lbs from the lbs of the 5$ mix, 240lbs, to get the number of lbs of the $3 mix, 160lbs.