High School

A store sells almonds for [tex]\$7[/tex] per pound, cashews for [tex]\$10[/tex] per pound, and walnuts for [tex]\$12[/tex] per pound. A customer buys 12 pounds of mixed nuts consisting of almonds, cashews, and walnuts for [tex]\$118[/tex]. The customer buys 2 more pounds of walnuts than cashews.

The matrix below represents this situation:

[tex]
\begin{array}{ccc|c}
0 & -1 & 1 & 2 \\
7 & 10 & 12 & 118 \\
1 & 1 & 1 & 12
\end{array}
[/tex]

If the reduced row echelon form of this matrix represents the amount of each type of nut the customer buys, which statement is a possible interpretation of the results?

A. The customer buys 1 more pound of walnuts than almonds and 1 more pound of almonds than cashews.

B. The customer buys 2 more pounds of walnuts than almonds and 2 more pounds of almonds than cashews.

C. The customer buys 0.5 more pounds of walnuts than almonds and 2.5 more pounds of almonds than cashews.

D. The customer buys 6.5 more pounds of walnuts than almonds and 8.5 more pounds of almonds than cashews.

Answer :

We first introduce variables for the amounts (in pounds) of each type of nut purchased:

- Let [tex]$a$[/tex] be the pounds of almonds.
- Let [tex]$c$[/tex] be the pounds of cashews.
- Let [tex]$w$[/tex] be the pounds of walnuts.

From the problem, we have the following three conditions:

1. The customer buys 2 more pounds of walnuts than cashews:
[tex]$$w - c = 2.$$[/tex]

2. The total cost is \[tex]$118. With almonds at \$[/tex]7 per pound, cashews at \[tex]$10 per pound, and walnuts at \$[/tex]12 per pound:
[tex]$$7a + 10c + 12w = 118.$$[/tex]

3. The total weight of the nuts is 12 pounds:
[tex]$$a + c + w = 12.$$[/tex]

Step 1. Express one variable in terms of another.

From the first equation,
[tex]$$w = c + 2.$$[/tex]

Step 2. Substitute [tex]$w$[/tex] into the total weight equation.

Replace [tex]$w$[/tex] in the equation [tex]$a + c + w = 12$[/tex]:
[tex]$$a + c + (c + 2) = 12.$$[/tex]
Combine like terms:
[tex]$$a + 2c + 2 = 12.$$[/tex]
Subtract 2 from both sides:
[tex]$$a + 2c = 10.$$[/tex]
Solve for [tex]$a$[/tex]:
[tex]$$a = 10 - 2c.$$[/tex]

Step 3. Substitute [tex]$a$[/tex] and [tex]$w$[/tex] into the cost equation.

The cost equation is:
[tex]$$7a + 10c + 12w = 118.$$[/tex]
Substitute [tex]$a = 10 - 2c$[/tex] and [tex]$w = c + 2$[/tex]:
[tex]$$7(10 - 2c) + 10c + 12(c + 2) = 118.$$[/tex]
Expand the terms:
[tex]$$70 - 14c + 10c + 12c + 24 = 118.$$[/tex]
Combine like terms:
[tex]$$70 + 24 + (-14c + 10c + 12c) = 118,$$[/tex]
[tex]$$94 + 8c = 118.$$[/tex]
Subtract 94 from both sides:
[tex]$$8c = 24.$$[/tex]
Divide both sides by 8:
[tex]$$c = 3.$$[/tex]

Step 4. Find [tex]$a$[/tex] and [tex]$w$[/tex].

Substitute [tex]$c = 3$[/tex] into [tex]$a = 10 - 2c$[/tex]:
[tex]$$a = 10 - 2(3) = 10 - 6 = 4.$$[/tex]
Substitute [tex]$c = 3$[/tex] into [tex]$w = c + 2$[/tex]:
[tex]$$w = 3 + 2 = 5.$$[/tex]

Step 5. Interpret the results.

The customer buys:
- 4 pounds of almonds,
- 3 pounds of cashews,
- 5 pounds of walnuts.

Now, compute the differences:
- Difference between walnuts and almonds:
[tex]$$w - a = 5 - 4 = 1.$$[/tex]
- Difference between almonds and cashews:
[tex]$$a - c = 4 - 3 = 1.$$[/tex]

This means the customer purchases 1 more pound of walnuts than almonds, and 1 more pound of almonds than cashews.

Conclusion:

The results correspond to the statement:

"The customer buys 1 more pound of walnuts than almonds and 1 more pound of almonds than cashews."

Thus, the correct interpretation is option 1.