Algebra II : Setting Up Equations

Study concepts, example questions & explanations for Algebra II

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Example Questions

Example Question #1 : Setting Up Equations

Tom is painting a fence \(\displaystyle 100\) feet long. He starts at the West end of the fence and paints at a rate of \(\displaystyle 5\) feet per hour. After \(\displaystyle 2\) hours, Huck joins Tom and begins painting from the East end of the fence at a rate of \(\displaystyle 8\) feet per hour. After \(\displaystyle 2\) hours of the two boys painting at the same time, Tom leaves Huck to finish the job by himself.

If Huck completes painting the entire fence after Tom leaves, how many more hours will Huck work than Tom?

Possible Answers:

\(\displaystyle 5\ hours\)

\(\displaystyle 6\ hours\)

\(\displaystyle 4\ hours\)

\(\displaystyle 10\ hours\)

\(\displaystyle 3\ hours\)

Correct answer:

\(\displaystyle 6\ hours\)

Explanation:

Tom paints for a total of \(\displaystyle 4\) hours (2 on his own, 2 with Huck's help). Since he paints at a rate of \(\displaystyle 5\) feet per hour, use the formula

\(\displaystyle distance = rate \times time\) (or \(\displaystyle d = rt\))

to determine the total length of the fence Tom paints.

\(\displaystyle d = (5)(4)\)

\(\displaystyle d = 20\) feet

Subtracting this from the total length of the fence \(\displaystyle 100\) feet gives the length of the fence Tom will NOT paint: \(\displaystyle 100 - 20 = 80\) feet. If Huck finishes the job, he will paint that \(\displaystyle 80\) feet of the fence. Using \(\displaystyle d = rt\), we can determine how long this will take Huck to do:

\(\displaystyle 80 = 8(t)\)

\(\displaystyle t = 10\) hours.

If Huck works \(\displaystyle 10\) hours and Tom works \(\displaystyle 4\) hours, he works \(\displaystyle 6\) more hours than Tom.

 

 

 

 

Example Question #1 : Setting Up Equations

If the roots of a function are \(\displaystyle x=5,-4\), what does the function look like in \(\displaystyle ax^{2}+bx+c=0\) format?

Possible Answers:

\(\displaystyle x^{2}+x+20=0\)

\(\displaystyle x^{2}-x-20=0\)

\(\displaystyle x^{2}-9x-20=0\)

No equation of this form is possible.

\(\displaystyle x^{2}+x-20=0\)

Correct answer:

\(\displaystyle x^{2}-x-20=0\)

Explanation:

This is a FOIL problem. First, we must set up the problem in a form we can use to create the function. To do this we take the opposite sign of each of the numbers and place them in this format: \(\displaystyle (x-5)(x+4)\).

Now we can FOIL.

First: \(\displaystyle x^2\)

Outside: \(\displaystyle 4x\)

Inside: \(\displaystyle -5x\)

Last: \(\displaystyle -20\)

Then add them together to get \(\displaystyle x^{2}+4x-5x-20\).

Combine like terms to find the answer, which is \(\displaystyle x^{2}-x-20=0\).

Example Question #1 : Setting Up Equations

Two numbers have a ratio of 5:6 and half of their sum is 22. What are the numbers?

Possible Answers:

\(\displaystyle 18,26\)

\(\displaystyle 28,16\)

\(\displaystyle 12,32\)

\(\displaystyle 20,24\)

\(\displaystyle 30,14\)

Correct answer:

\(\displaystyle 20,24\)

Explanation:

Set up the equation:

\(\displaystyle \frac{5x + 6x}{2} = 22\)

Solve the equation:

\(\displaystyle 5x + 6x = 44\)

\(\displaystyle 11x = 44\)

\(\displaystyle x=4\)

Find the two numbers:

The two numbers have a ratio of 5:6, therefore the ratio can also be represented as:

\(\displaystyle 5x:6x\)

\(\displaystyle 5(4):6(4)\)

\(\displaystyle 20,24\)

The two numbers are 20 and 24.

Example Question #352 : Basic Single Variable Algebra

Set up an equation that properly displays the information given.

Suzanne has a pack of multi-colored jelly beans. She wants to sort them into equal amounts to give out to her four friends, but not until she eats eight of them. If the total pack contains 60 jelly beans, then how many is each friend going to get?

Possible Answers:

\(\displaystyle 60/x-8=4\)

\(\displaystyle 60x/4=15\)

\(\displaystyle 60/4x=8\)

\(\displaystyle 4\)\(\displaystyle x-8=60\)

\(\displaystyle 4x+8=60\)

Correct answer:

\(\displaystyle 4x+8=60\)

Explanation:

Let \(\displaystyle x\) be the number of jelly beans that each friend will receive. She has four friends, so the total number of jelly beans her friends will receive is \(\displaystyle 4x\). Suzanne eats another eight, so the equation can be written as \(\displaystyle 4x+8=60\).

Example Question #2 : Setting Up Equations

What is the equation of the line that has a slope of 3 and passes through the point (3,-6)?

Possible Answers:

\(\displaystyle y=-3x-9\)

\(\displaystyle y=3x-3\)

\(\displaystyle y=-3x-15\)

\(\displaystyle y=-3x-11\)

\(\displaystyle y=3x-15\)

Correct answer:

\(\displaystyle y=3x-15\)

Explanation:

The equation for a line in slope-intercept form is:

\(\displaystyle y=m(x-x_{1})+y_{1}\)

where \(\displaystyle x_{1}\) and \(\displaystyle y_{1}\) are the known coordinates (3,-6).

Substituting gives

\(\displaystyle y=3(x-3)-6\)

and simplifying gives the final answer:

\(\displaystyle y=3x-15\)

Example Question #3 : Setting Up Equations

Your friend goes on a diet to lose a little weight. He starts at \(\displaystyle 180\) pounds and cuts his calories by \(\displaystyle 500\) a day. Write a linear equation to express your friend's weight in pounds as a function of weeks on the diet. Hint: there are \(\displaystyle 3\),\(\displaystyle 500\) calories in a pound.

Possible Answers:

\(\displaystyle y=x+180\)

\(\displaystyle y=-x+180\)

\(\displaystyle y=7x+180\)

\(\displaystyle y=\frac{1}{7}x-180\)

\(\displaystyle y=x-180\)

Correct answer:

\(\displaystyle y=-x+180\)

Explanation:

The question asks for a relation between pounds lost and weeks on the diet. If each day your friend cuts 500 calories, the number of pounds he is losing per week is 1:

\(\displaystyle \frac{500 \text{ cal}}{\text{ day}}\cdot \frac{7\text{ days}}{\text{ week}}\cdot \frac{1\text{ pound}}{3500 \text{ cal}}=\frac{1 \text{ pound}}{ \text{ week}}\)

The rate of change, or slope, is therefore -1. The slope is negative because the independent variable (weight in pounds) is decreases as the dependent variable (time in weeks) increases. The y-intercept is 180, because that is how much your friend weighs at the start, when time = 0. Plugging these values into \(\displaystyle y=mx+b\) form, we end up with: 

\(\displaystyle y=-1x+180\)

Example Question #3 : Setting Up Equations

A circular tower stands surrounded by a circular moat. A bridge provides a passage over the moat to the tower. The distance from the outer edge of the moat to the center of the tower is \(\displaystyle 100\) meters. The area of the floor of tower is \(\displaystyle 4900\pi\) \(\displaystyle m^{2}\). How long is the bridge over the moat?

Possible Answers:

\(\displaystyle 300 m\)

\(\displaystyle 700 m\)

\(\displaystyle 30 m\)

\(\displaystyle 70 m\)

\(\displaystyle 50 m\)

Correct answer:

\(\displaystyle 30 m\)

Explanation:

The distance from the outer edge of the moat to the center of the castle is the radius (100 m) of the larger circle formed by the outer edge of the circular moat.

The radius of the tower's floor (found using the area of the floor), needs to be subracted from 100 m to find the distance fo the bridge. 

\(\displaystyle \sqrt{4900}=70\)

\(\displaystyle 100m - 70m = 30 m\)

Example Question #3 : Equations

The monthly cost to insure your cars varies directly with the number of cars you own. Right now, you are paying $420 per month to insure 3 cars, but you plan to get 2 more cars, so that you will own 5 cars. How much does it cost to insure 5 cars monthly?

Possible Answers:

\(\displaystyle \$748\)

\(\displaystyle \$810\)

\(\displaystyle \$700\)

\(\displaystyle \$633\)

\(\displaystyle \$390\)

Correct answer:

\(\displaystyle \$700\)

Explanation:

The statement, 'The monthly costly to insure your cars varies directly with the number of cars you own' can be mathematically expressed as \(\displaystyle M=kC\). M is the monthly cost, C is the number of cars owned, and k is the constant of variation.

Given that it costs $420 a month to insure 3 cars, we can find the k-value.

\(\displaystyle 420=k * 3\)

Divide both sides by 3.

\(\displaystyle k=140\)

Now, we have the mathematical relationship.

\(\displaystyle M=140C\)

Finding how much it costs to insure 5 cars can be found by substituting 5 for C and solving for M.

\(\displaystyle M=140(5)\)

\(\displaystyle M=700\)

Example Question #4 : Setting Up Equations

Express as an equation. 

\(\displaystyle 20\) more than \(\displaystyle x\) is \(\displaystyle 40\)

Possible Answers:

\(\displaystyle 20=40+x\)

\(\displaystyle x+20=40\)

\(\displaystyle x+40=20\)

\(\displaystyle 20x=40\)

\(\displaystyle 20x>40\)

Correct answer:

\(\displaystyle x+20=40\)

Explanation:

Take every word and translate into math. 

\(\displaystyle 20\) more than means that you need to add \(\displaystyle 20\) to something which is \(\displaystyle x\).

Anytime you see "is" means equal.

Now let's combine and create an expression of \(\displaystyle x+20=40.\)

Example Question #9 : Setting Up Equations

Express as an equation.

\(\displaystyle 2\) times \(\displaystyle x\) is \(\displaystyle 3\) less than \(\displaystyle x\)

Possible Answers:

\(\displaystyle 3x=x-2\)

\(\displaystyle 2x=x-3\)

\(\displaystyle 2x>x-3\)

\(\displaystyle x=2x-3\)

\(\displaystyle 2x< x-3\)

Correct answer:

\(\displaystyle 2x=x-3\)

Explanation:

Take every word and translate into math.

\(\displaystyle 2\) times something means that you need to multiply \(\displaystyle 2\) to something which is \(\displaystyle x\)

\(\displaystyle 3\) less than means that you need to subtract \(\displaystyle 3\) from \(\displaystyle x\).  

Anytime you see "is" means equal.

Let's combine to get \(\displaystyle 2x=x-3\)

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