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]
\[
\begin{bmatrix}
0 & -1 & 1 & 2 \\
7 & 10 & 12 & 118 \\
1 & 1 & 1 & 12
\end{bmatrix}
\]
[/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 pound 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 :

Let the number of pounds of almonds, cashews, and walnuts be [tex]$a$[/tex], [tex]$c$[/tex], and [tex]$w$[/tex], respectively. We are given the following information:

1. The total weight is 12 pounds:
[tex]$$a + c + w = 12.$$[/tex]
2. The total cost is \[tex]$118, with prices \$[/tex]7, \[tex]$10, and \$[/tex]12 per pound for almonds, cashews, and walnuts, respectively:
[tex]$$7a + 10c + 12w = 118.$$[/tex]
3. There are 2 more pounds of walnuts than cashews:
[tex]$$w - c = 2.$$[/tex]

Step 1. Express [tex]$w$[/tex] in terms of [tex]$c$[/tex].
From the third equation,
[tex]$$w = c + 2.$$[/tex]

Step 2. Substitute [tex]$w = c + 2$[/tex] into the total weight equation.
The first equation becomes:
[tex]\[
a + c + (c + 2) = 12.
\][/tex]
Simplify the equation:
[tex]\[
a + 2c + 2 = 12 \quad \Longrightarrow \quad a + 2c = 10.
\][/tex]
Solve for [tex]$a$[/tex]:
[tex]\[
a = 10 - 2c.
\][/tex]

Step 3. Substitute [tex]$a = 10 - 2c$[/tex] and [tex]$w = c + 2$[/tex] into the cost equation.
The cost equation becomes:
[tex]\[
7(10 - 2c) + 10c + 12(c + 2) = 118.
\][/tex]
Expand and simplify:
[tex]\[
70 - 14c + 10c + 12c + 24 = 118.
\][/tex]
Combine like terms:
[tex]\[
70 + 24 + (-14c + 10c + 12c) = 118 \quad \Longrightarrow \quad 94 + 8c = 118.
\][/tex]
Solve for [tex]$c$[/tex]:
[tex]\[
8c = 118 - 94 \quad \Longrightarrow \quad 8c = 24 \quad \Longrightarrow \quad c = 3.
\][/tex]

Step 4. Determine [tex]$a$[/tex] and [tex]$w$[/tex].
Using [tex]$a = 10 - 2c$[/tex]:
[tex]\[
a = 10 - 2(3) = 10 - 6 = 4.
\][/tex]
Using [tex]$w = c + 2$[/tex]:
[tex]\[
w = 3 + 2 = 5.
\][/tex]

Step 5. Calculate the differences between the quantities.
Difference between walnuts and almonds:
[tex]\[
w - a = 5 - 4 = 1.
\][/tex]
Difference between almonds and cashews:
[tex]\[
a - c = 4 - 3 = 1.
\][/tex]

Conclusion:
The customer buys 1 more pound of walnuts than almonds and 1 more pound of almonds than cashews. This interpretation corresponds to the statement:

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