Percentages – Outschool

Percentages Lesson

I. Simple Percentage

Ex. Johnny owns 253 of the 278 Komiji playing cards. What percentage of the cards does he own?

Step 1: Divide the number of cards he owns by the total number of Komiji playing cards.

253/278 = .910

Step 2: Move the decimal two places to the right: .910 = 91.0%.

Johnny owns 91% of the Komiji playing cards.

II. Multiple Percentage Increases and Decreases

Ex. A sweater is marked down by 10%. The new price is what percent of the original price?

Original Price · .9 (because 90% of the price is remaining)

The new price is 90% of the original price.

Ex. A sweater is marked down by 10% for three consecutive weeks. The new price is what percent of the original price?

***Be prepared: Three consecutive 10% decreases does not equal a 30% decrease.***

Original Price · .9 · .9 · .9

→ Original Price · .729

The new price is 72.9% of the original price.

Ex. A house’s value increased by 20% for YEAR 1 and another 15% in YEAR 2. By what percentage has the house’s value increased over the two years?

Original House Value · 1.2 · 1.15 (because house values were 120% and 115% of previous value)

→ Original House Value · 1.38

The house’s value is now 138% of its original value. The house’s value has increased by 38% over the two years.

III. Equations with Percentages

Be prepared to convert a written statement into the equivalent algebraic form, and then solve!

Ex. 20% of a number is 120. What is that number?

.2 · x = 120 (Turn the words into an equation. Remember “of” in math means multiply.)

x = 600 (After dividing by .2)

The number is 600.