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 x = to pounds of nuts and y = pounds of raisins

We have two unknowns in the problem therefore we need two equations in order to solve the problem.

Pounds of trail mix = Pounds of nuts + Pounds of raisins
60 = x + y > First equation

Cost/lb (lb) = cost of nut/lb (lb) + cost of raisins/lb (lb)
5(60) = 9(x) + 3(y)
300 = 9x + 3y > Second equation

Therefore, 
x = 20 lb of nuts
y = 40 lb raisins