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One-Step Equations Transformed by Multiplication/Division

Multiplication or division to solve an equation.

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One-Step Equations Transformed by Multiplication-Division

What if you had an algebraic equation involving multiplication or division like \begin{align*}-5x = 3\end{align*}5x=3? How could you solve it for the unknown variable x? After completing this Concept, you'll be able to solve equations like this one.

Watch This

CK-12 Foundation: 0302S Solving Equations with Multiplication and Division (H264)

Guidance

License: CC BY-NC 3.0

Suppose you are selling pizza for $1.50 a slice and you can get eight slices out of a single pizza. How much money do you get for a single pizza? It shouldn’t take you long to figure out that you get \begin{align*}8 \times \$1.50 = \$12.00\end{align*}8×$1.50=$12.00. You solved this problem by multiplying. Here’s how to do the same thing algebraically, using \begin{align*}x\end{align*}x to stand for the cost in dollars of the whole pizza.

Example A

Solve \begin{align*} \frac{1}{8} \cdot x = 1.5\end{align*}18x=1.5.

Our \begin{align*}x\end{align*} is being multiplied by one-eighth. To cancel that out and get \begin{align*}x\end{align*} by itself, we have to multiply by the reciprocal, 8. Don’t forget to multiply both sides of the equation.

\begin{align*}8 \left ( \frac{1}{8} \cdot x \right ) &= 8(1.5)\\ x &= 12\end{align*}

Example B

Solve \begin{align*}0.25x = 5.25\end{align*}.

0.25 is the decimal equivalent of one fourth, so to cancel out the 0.25 factor we would multiply by 4.

\begin{align*}4(0.25x) &= 4(5.25)\\ x &= 21\end{align*}

Solving by division is another way to isolate \begin{align*}x\end{align*}. Suppose you buy five identical candy bars, and you are charged $3.25. How much did each candy bar cost? You might just divide $3.25 by 5, but let’s see how this problem looks in algebra.

Example C

Solve \begin{align*}5x = 3.25\end{align*}.

To cancel the 5, we divide both sides by 5.

\begin{align*} \frac{5x}{5} &= \frac{3.25}{5}\\ x &= 0.65\end{align*}

Example D

Solve \begin{align*}1.375x = 1.2\end{align*}.

Divide by 1.375

\begin{align*} x &= \frac{1.2}{1.375}\\ x &= 0.8 \overline{72}\end{align*}

Notice the bar above the final two decimals; it means that those digits recur, or repeat. The full answer is 0.872727272727272....

To see more examples of one - and two-step equation solving, watch the Khan Academy video series starting at http://www.youtube.com/watch?v=bAerID24QJ0.

Watch this video for help with the Examples above.

CK-12 Foundation: Solving Equations with Multiplication and Division

Guided Practice

Solve:

a) \begin{align*} \frac{9x}{5} = 5\end{align*}.

b) \begin{align*}7x = \frac{5}{11}\end{align*}.

Solutions:

a) \begin{align*}\frac{9x}{5}\end{align*} is equivalent to \begin{align*} \frac{9}{5} \cdot x\end{align*}, so to cancel out that \begin{align*} \frac{9}{5}\end{align*}, we multiply by the reciprocal, \begin{align*} \frac{5}{9}\end{align*}.

\begin{align*} \frac{5}{9} \left ( \frac{9x}{5} \right ) &= \frac{5}{9}(5)\\ x &= \frac{25}{9}\end{align*}

b) Divide both sides by 7.

\begin{align*}x &= \frac{5}{11.7}\\ x &= \frac{5}{77} \end{align*}

Explore More

For 1-5, solve the following equations for \begin{align*}x\end{align*}.

  1. \begin{align*}7x = 21 \end{align*}
  2. \begin{align*}4x = 1 \end{align*}
  3. \begin{align*}\frac{5x}{12} = \frac{2}{3}\end{align*}
  4. \begin{align*}0.01x = 11\end{align*}
  5. \begin{align*}\frac{-2x}{9} = \frac{10}{3}\end{align*}

For 6-10, solve the following equations for the unknown variable.

  1. \begin{align*}21s = 3 \end{align*}
  2. \begin{align*}-7a = -5 \end{align*}
  3. \begin{align*}\frac{7f}{11} = \frac{7}{11} \end{align*}
  4. \begin{align*}6r = \frac{3}{8} \end{align*}
  5. \begin{align*}\frac{9b}{16} = \frac{3}{8} \end{align*}

Answers for Explore More Problems

To view the Explore More answers, open this PDF file and look for section 3.2. 

Image Attributions

  1. [1]^ License: CC BY-NC 3.0

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