# ACT Math : How to find an answer with a matrix

## Example Questions

### Example Question #1 : How To Find An Answer With A Matrix

With matrix notation, what does M2x3 x N3x4 equal?

P3x4

P3x3

P2x4

P4x2

None of the answers are correct

P2x4

Explanation:

M2x3 x N3x4 = P2x4

In general matrix notation, Mrxc shows that the matrix is named M and r is the number of rows and c is the number of columns.  When multiplying two matrices, the number of columns in the first matrix must match the number of rows in the second matrix.  In addition, when adding or subtracting matrices, the matrices must be of the same size.

### Example Question #1 : How To Find An Answer With A Matrix

What is the solution to the following matrix?

Explanation:

In order to solve the matrix, the determinant rule "ad-bc" must be used.  is in the "a" position,  is in the "b" position,  is in the "c" position, and  is in the "d" position. After plugging the numbers into "ad-bc," we get

### Example Question #2 : How To Find An Answer With A Matrix

Which of the following augmented matrices can be used to solve this system of equations?

Explanation:

To set up and augmented matrix for a 3x3 system of equations, all equations must be in standard form . The third equation is already in standard form; the first two are not and must be rewritten as such.

The system is now

Write the augmented matrix with each row comprising the coefficients of one equation in order:

is the correct choice.

### Example Question #2 : How To Find An Answer With A Matrix

Which of the following augmented matrices can be used to solve this system of equations?

Explanation:

To set up and augmented matrix for a 2x2 system of equations, both equations must be in standard form . The second equation is already in standard form.

Rewrite the first equation in standard form as follows:

The system has been rewritten as

Write the augmented matrix with each row comprising the coefficients of one equation in order:

is the correct choice.

### Example Question #3 : How To Find An Answer With A Matrix

A high school choir sold large boxes of cookies for $5.75 each, medium boxes for$4.75 each, and small boxes for $3.25 each. The band sold a total of 445 boxes and raised a total of$1,924.25. There were twenty more medium boxes sold than large boxes.

Which of the following augmented matrices represents the system of equations that could be set up to solve this problem?

Explanation:

If we let , and  represent the number of large, medium, and small boxes sold, respectively, since twenty more medium boxes than large were sold, one equation of the 3x3 system will be

or, in standard form,

Since 445 boxes were sold, another linear equation will be

The money raised from the sale of  large boxes of cookies, each of which cost $5.75, is ; the money raised from the sale of small boxes of cookies, each of which cost$4.75, is ; and the money raised from the sale of  small boxes of cookies, each of which cost $3.25, is The total money raised is$1,924.25, so the other linear equation of the system is

The augmented matrix of this system will comprise the coefficients of these equations, all of which are now standard form, so the matrix will be

,

which is the correct choice.

The barista at the Teahouse of the December Sun has a problem. He needs to mix twenty pounds of two different kinds of tea together to create a blend called Strawberry Peppermint Delight. The two varieties are Peppermint Nirvana, which costs $12 a pound, and Strawberry Fields, which costs$16 a pound; the new tea will cost $13 a pound, and it will sell for the same price as the two blended teas would separately. How much of each variety will go into the twenty pounds of Strawberry Peppermint Delight? Which of the following augmented matrices represents the system of equations that could be set up to solve this problem? Possible Answers: Correct answer: Explanation: If the barista mixes pounds of Peppermint Nirvana and pounds of Strawberry Fields to make twenty pounds of tea total, then will be one of the equations in the system. pounds of Peppermint Nirvana tea for$12 a pound will cost a total of  dollars;  pounds of Strawberry Fields tea will cost a total of  dollars. Tewnty pounds of the Strawberry Peppermint Delight tea for $13 a pound will cost dollars. Since the tea will sell for the same price blended as separate, the other equation of the system will be The augmented matrix of this system will comprise the coefficients of these equations, both of which are in standard form, so the matrix will be . ### Example Question #5 : How To Find An Answer With A Matrix Read the following question: A high school band sold large boxes of cookies for$4.75 each and small boxes of cookies for $3.25 each. The band sold a total of 305 boxes and raised a total of$1,196.75.

Which of the following augmented matrices represents the system of equations that could be set up to solve this problem?

Explanation:

If we let  and  represent the number of large and small boxes sold, respectively, since 305 boxes were sold, one linear equation of the 2x2 system will be

The money raised from the sale of  large boxes of cookies, each of which cost $4.75, is ; the money raised from the sale of small boxes of cookies, each of which cost$3.25, is . The total money raised is \$1,196.75, so the other linear equation of the system is

The augmented matrix of this system will comprise the coefficients of these equations, both of which are in standard form, so the matrix will be

### Example Question #5 : How To Find An Answer With A Matrix

A chemist needs one liter of a solution of 20% alcohol for an experiment. However, he only has two solutions on hand, one of which is 10% alcohol and one of which is 40% alcohol. How much of each solution must he mix in order to make his desired solution?

Which of the following augmented matrices represents the system of equations that could be set up to solve this problem?

Explanation:

If we let  and  represent the amount of the weaker and stronger solutions, respectively, since the chemist needs 1 liter of the resulting solution, one linear equation of the 2x2 system will be

The amount of alcohol in  liters of a 10% solution will be ; the amount of alcohol in  liters of a 40% solution will be ; and the total amount of alcohol in the resulting solution will be 20 % of a liter, or 0.20 liters. Therefore, the second linear equation of the system will be

The augmented matrix of this system will comprise the coefficients of these equations, both of which are in standard form, so the matrix will be

,

which is the correct choice.

### Example Question #6 : How To Find An Answer With A Matrix

Below is a matrix of the items in Jon's wardrobe:

How many blue items does Jon own?

Explanation:

To find the number of blue items Jon has, we sum the entries in the column labelled  and get . Recall that matrices are organized by rows and columns where each entry refers to the number of items that are, in this case, both of the same color AND type. All the entries in any one column are of the same color. All of the entries in any row are of the same clothing type.

### Example Question #3 : How To Find An Answer With A Matrix

Below is a matrix of the items in Jon's wardrobe:

How many combinations of blue pants and red shirts can Jon wear for the upcoming 4th of July party?