Question

1. Stephanie would like to make a 5 lb nut mixture that is 60% peanuts and 40% almonds. She has several pounds of peanuts and several pounds of a mixture that is 20% peanuts and 80% almonds. Let p represent the number of pounds of peanuts needed to make the new mixture, and let m represent the number of pounds of the 80% almond-20% peanut mixture.
(a) What is the system that models this situation?
(b) Which of the following is a solution to the system: 2 lb peanuts and 3 lb mixture; 2.5 lb peanuts and 2.5 lb mixture; 4 lb peanuts and 1 lb mixture? Show all your work.

Answer 1

Answers:

(a) p + m = 5
     0.8m = 2

(b) 2.5 lb peanuts and 2.5 lb mixture

Explanations:

(a) Note that we just need to mix the following to get the desired mixture:

     - peanut (p) - peanuts whose amount is p
     - mixture (m) - mixture (80% almonds and 20% peanuts) that has an amount of m; we denote this as

By mixing the peanuts (p) and the mixture (m), we combine their weights and equate it 5 since the mixture has a total of 5 lb.

Hence, 

p + m = 5

Note that the desired 5-lb mixture has 40% almonds. Thus, the amount of almonds in the desired mixture is 2 lb (40% of 5 lb, which is 0.4 multiplied by 5).

Moreover, since the mixture (m) has 80% almonds, the weight of almonds that mixture is 0.8m.

Since we mix mixture (m) with the pure peanut to get the desired mixture, the almonds in the desired mixture are also the almonds in the mixture (m). 
So, we can equate the amount of almonds in mixture (m) to the amount of almonds in the desired measure.

In terms mathematical equation,

0.8m = 2 

Hence, the system of equations that models the situation is 

p + m = 5
0.8m = 2

(b) To solve the system obtained in (a), we first label the equations for easy reference,

(1) p + m = 5
(2) 0.8m = 2

Note that using equation (2), we can solve the value of m by dividing both sides of (2) by 0.8. By doing this, we have

m = 2.5

Then, we substitute the value of m to equation (1) to solve for p:

p + m = 5
p + 2.5 = 5   (3)

To solve for p, we subtract both sides of equation (3) by 2.5. Thus,

p = 2.5

Hence, 

m = 2.5, p = 2.5

Therefore, the solution to the system is 2.5 lb peanuts and 2.5 lb mixture.