How many pounds of nuts selling for $9 per lb and raisins selling for $3 per lb should a person combine to obtain 60 lb of a trail mix selling for $5 per lb

Let n be the number of lbs of nuts.
Let r be the number of lbs of raisins.
9n + 3r = 60 * 5 = 300 (1)
n + r = 60
n = 60 - r (2)
Plugging the expression for n in (2) into (1) we get:
9(60 - r) + 3r = 300
which expands to:
540 - 9r + 3r = 300 (3)
Simplifying and rearranging (3) we get:
-6r = -240
Therefore r, the weight of raisins = 40 lbs
and the weight of nuts is 60 - 40 = 20 lbs

The variables should add up to equal 1