How much cashews at 1.58 per pound must be mixed with walnuts that cost 78 cents per pound to make a mixture of 40 pounds of nuts that cost 1.50 per pound?

Let x be the weight of cashews needed.

Since the total weight of the mixture is 40 pounds, the weight of walnuts is 40 - x pounds.

The cost of the mixture is $1.50 per pound, so the cost equation is:

1.58x + 0.78(40-x) = 1.50(40)

1.58x + 31.2 - 0.78x = 60

0.80x = 28.8

x = 36

Therefore, 36 pounds of cashews at $1.58 per pound must be mixed with 4 pounds of walnuts at $0.78 per pound to make a mixture of 40 pounds of nuts that cost $1.50 per pound.