3.7: Calculating Percentages
- Page ID
- 51001
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(Mental Math verses pencil to paper)
30 is 20% of what number?
Mental math version: We know that a whole amount is 100%, so five 20%s are 100%. Therefore, we can multiply 30 by 5 and get 150, our answer.
But sometimes, it is not that easy. We need a formula:
\[\dfrac{\color {Red}\textit{is}}{\color {Blue}\textit{of}}=\dfrac{\%}{100} \nonumber \]
30 is 23% of what number? Use the proportion below, cross multiply, and solve for \(x\). (Algebra)
\[\dfrac{\color {Red}30}{\color {Blue}x}=\dfrac{23}{100} \nonumber \]
Partner Activity 1
- Find 16% of 54.
- What percent of 97 is 43?
- 21% of what number is 842.
Practice Problems
- Find 94% of 329.
- What percent of 14 is 13? (round to the nearest hundredth)
- 64% of what number is 73. (round to the nearest hundredth)