Final answer:
To get a mixture of coffee worth $5 a pound, you should add 30 pounds of coffee worth $7 a pound to 20 pounds of coffee worth $2 a pound.
Step-by-step explanation:
To solve this problem, we can use the method of mixtures. Let's assume that x pounds of coffee worth $7 a pound is added to 20 pounds of coffee worth $2 a pound. The total weight of the mixture will be (x + 20) pounds. The total value of the mixture will be the sum of the values of the two types of coffee.
The value of the 20 pounds of coffee worth $2 a pound is 20 * 2 = $40. The value of the x pounds of coffee worth $7 a pound is 7x. So, the total value of the mixture will be $40 + 7x.
Since we want the mixture to be worth $5 a pound, we can set up the equation:
(40 + 7x) / (20 + x) = 5
To solve this equation, we can cross-multiply:
(40 + 7x) = 5(20 + x)
Expanding the right side of the equation, we get:
40 + 7x = 100 + 5x
Subtracting 5x from both sides, we get:
2x = 60
Dividing both sides by 2, we get:
x = 30
Therefore, 30 pounds of coffee worth $7 a pound should be added to 20 pounds of coffee worth $2 a pound to get a mixture worth $5 a pound.