ACT Math : How to find the solution to an inequality with division

Study concepts, example questions & explanations for ACT Math

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Example Questions

Example Question #1 : How To Find The Solution To An Inequality With Division

Which of the following inequalities defines the solution set for the inequality 14 – 3x ≤ 5?

Possible Answers:

x ≤ –3

x ≥ 3

x ≤ 3

x ≥ –3

x ≤ –19/3

Correct answer:

x ≥ 3

Explanation:

To solve this inequality, you should first subtract 14 from both sides.

This leaves you with –3x ≤ –9.

In the next step, you divide both sides by –3, remembering to flip the inequality sign when you do this.

This leaves you with the solution x ≥ 3.

If you selected x ≤ 3, you probably forgot to flip the sign. If you selected one of the other solutions, you may have subtracted incorrectly.

Example Question #2 : How To Find The Solution To An Inequality With Division

Solve 6x – 13 > 41

Possible Answers:

< 9

> 6

> 9

> 4.5

< 6

Correct answer:

> 9

Explanation:

Add 13 to both sides, giving you 6x > 54, divide both sides by 6, leaving > 9.

Example Question #1 : How To Find The Solution To An Inequality With Division

Solve for \(\displaystyle x\).

\small 14-2x\geq 22\(\displaystyle \small 14-2x\geq 22\)

Possible Answers:

\small x\leq4\(\displaystyle \small x\leq4\)

\small x\leq-4\(\displaystyle \small x\leq-4\)

\small x\geq-4\(\displaystyle \small x\geq-4\)

\small x\geq4\(\displaystyle \small x\geq4\)

Correct answer:

\small x\leq-4\(\displaystyle \small x\leq-4\)

Explanation:

\small 14-2x\geq22\(\displaystyle \small 14-2x\geq22\)

\small -2x\geq8\(\displaystyle \small -2x\geq8\)

When dividing both sides of an inequality by a negative number, you must change the direction of the inequality sign.

\small x\leq-4\(\displaystyle \small x\leq-4\)

Example Question #3 : How To Find The Solution To An Inequality With Division

Solve 3 < 5x + 7

Possible Answers:

\(\displaystyle -\frac{4}{5}>x\)

\(\displaystyle -\frac{3}{5}< x\)

2 > x

\(\displaystyle -\frac{4}{5}< x\)

\(\displaystyle 2< x\)

Correct answer:

\(\displaystyle -\frac{4}{5}< x\)

Explanation:

Subtract seven from both sides, then divide both sides by 5, giving you –4/5 < x.

Example Question #1 : How To Find The Solution To An Inequality With Division

Find is the solution set for x where:

\(\displaystyle \left | 5x-3\right |>12\)

Possible Answers:

\(\displaystyle x> 3\) or \(\displaystyle x< -\frac{9}{5}\)

\(\displaystyle -3< x< \frac{9}{5}\)

\(\displaystyle -\frac{9}{5}< x< 3\)

\(\displaystyle x> -3\) or \(\displaystyle x< \frac{9}{5}\)

\(\displaystyle x< -\frac{9}{5}\)

Correct answer:

\(\displaystyle x> 3\) or \(\displaystyle x< -\frac{9}{5}\)

Explanation:

We start by splitting this into two inequalities, \(\displaystyle (5x-3)>12\) and \(\displaystyle (5x-3)< -12\) 

We solve each one, giving us \(\displaystyle x>3\) or \(\displaystyle x< \frac{-9}{5}\right\).

Example Question #161 : Algebra

Which of the following could be a value of \(\displaystyle x\), given the following inequality?

\(\displaystyle 8-7x\leq11x+4\)

Possible Answers:

\(\displaystyle -\frac{1}{3}\)

\(\displaystyle \frac{4}{9}\)

\(\displaystyle \frac{1}{9}\)

\(\displaystyle -\frac{2}{9}\)

\(\displaystyle -\frac{2}{3}\)

Correct answer:

\(\displaystyle \frac{4}{9}\)

Explanation:

The inequality that is presented in the problem is:

\(\displaystyle 8-7x\leq11x+4\)

Start by moving your variables to one side of the inequality and all other numbers to the other side:

\(\displaystyle -7x-11x\leq4-8\)

\(\displaystyle -18x\leq-4\)

Divide both sides of the equation by \(\displaystyle -18\). Remember to flip the direction of the inequality's sign since you are dividing by a negative number!

\(\displaystyle x\geq\frac{-4}{-18}\)

Reduce:

\(\displaystyle x\geq\frac{2}{9}\)

The only answer choice with a value greater than \(\displaystyle \frac{2}{9}\) is \(\displaystyle \frac{4}{9}\).

Example Question #1 : Inequalities

Solve for the \(\displaystyle y\)-intercept:

3y+11\geq 5y+6x-1\(\displaystyle 3y+11\geq 5y+6x-1\)

Possible Answers:

\(\displaystyle -6\)

\(\displaystyle -12\)

\(\displaystyle 6\)

\(\displaystyle 3\)

\(\displaystyle -3\)

Correct answer:

\(\displaystyle 6\)

Explanation:

Don't forget to switch the inequality direction if you multiply or divide by a negative.

3y+11\geq 5y+6x-1\(\displaystyle 3y+11\geq 5y+6x-1\)

-2y+11\geq6x-1\(\displaystyle -2y+11\geq6x-1\)

-2y\geq6x-12\(\displaystyle -2y\geq6x-12\)

-\frac{1}{2}(-2y\geq 6x-12)\(\displaystyle -\frac{1}{2}(-2y\geq 6x-12)\)

y\leq -3x+6\(\displaystyle y\leq -3x+6\)

Now that we have the equation in slope-intercept form, we can see that the y-intercept is 6.

Example Question #2 : Inequalities

Solve for \(\displaystyle x\):

\(\displaystyle -4x - 12 < 15\)

Possible Answers:

\(\displaystyle x < \frac{27}{4}\)

\(\displaystyle x< \frac{15}{4}\)

\(\displaystyle x > \frac{15}{4}\)

\(\displaystyle x < 15-4x\)

\(\displaystyle x > -\frac{27}{4}\)

Correct answer:

\(\displaystyle x > -\frac{27}{4}\)

Explanation:

Begin by adding \(\displaystyle 12\) to both sides, this will get the variable isolated:

\(\displaystyle -4x < 15 + 12\)

Or...

\(\displaystyle -4x < 27\)

Next, divide both sides by \(\displaystyle -4\):

\(\displaystyle x > -\frac{27}{4}\)

Notice that when you divide by a negative number, you need to flip the inequality sign!

Example Question #3 : Inequalities

Each of the following is equivalent to  

xy/z * (5(x + y))  EXCEPT:

 

Possible Answers:

5x² + y²/z

xy(5x + 5y)/z

xy(5y + 5x)/z

5x²y + 5xy²/z

Correct answer:

5x² + y²/z

Explanation:

Choice a is equivalent because we can say that technically we are multiplying two fractions together: (xy)/z and (5(x + y))/1.  We multiply the numerators together and the denominators together and end up with xy (5x + 5y)/z.  xy (5y + 5x)/z is also equivalent because it is only simplifying what is inside the parentheses and switching the order- the commutative property tells us this is still the same expression.  5x²y + 5xy²/z is equivalent as it is just a simplified version when the numerators are multiplied out.  Choice 5x² + y²/z is not equivalent because it does not account for all the variables that were in the given expression and it does not use FOIL correctly.

Example Question #5 : How To Find The Solution To An Inequality With Division

What is the solution set of the inequality \dpi{100} \small 3x+8<35\(\displaystyle \dpi{100} \small 3x+8< 35\) ?

Possible Answers:

\dpi{100} \small x>27\(\displaystyle \dpi{100} \small x>27\)

\dpi{100} \small x>9\(\displaystyle \dpi{100} \small x>9\)

\dpi{100} \small x<27\(\displaystyle \dpi{100} \small x< 27\)

\dpi{100} \small x<35\(\displaystyle \dpi{100} \small x< 35\)

\dpi{100} \small x<9\(\displaystyle \dpi{100} \small x< 9\)

Correct answer:

\dpi{100} \small x<9\(\displaystyle \dpi{100} \small x< 9\)

Explanation:

We simplify this inequality similarly to how we would simplify an equation

\dpi{100} \small 3x+8-8<35-8\(\displaystyle \dpi{100} \small 3x+8-8< 35-8\)

\dpi{100} \small \frac{3x}{3}<\frac{27}{3}\(\displaystyle \dpi{100} \small \frac{3x}{3}< \frac{27}{3}\)

Thus \dpi{100} \small x<9\(\displaystyle \dpi{100} \small x< 9\)

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