Percents
Lessons in this chapter
About Percents
Percents takes the ratio and proportion skills from the previous chapter and applies them to one specific, everyday ratio: a comparison out of 100.
It starts with relate fractions, decimals, and percents, converting fluidly between all three forms of the same value — 3/4, 0.75, and 75% are all the same number, just written differently — before find the part, percent, and whole of a quantity has kids solve for whichever one of those three pieces is missing from a percent problem.
Find the percent change of a quantity moves from a static percent to a comparison over time — calculating what percent a quantity increased or decreased by, like a price that rose from $40 to $50.
Solve percent problems involving discounts, markups, and tips applies percent change directly to money situations kids actually encounter — a 20% discount, a restaurant tip, a store’s markup — before apply the simple interest formula closes the chapter with I = Prt, calculating interest earned or owed based on a principal amount, rate, and time.
Frequently asked questions
A percent is a ratio out of 100 — 75% literally means 75 out of every 100. Relate Fractions, Decimals, and Percents has kids see that 75%, 3/4, and 0.75 are three different names for the exact same value.
To go from a fraction to a percent, divide the numerator by the denominator and multiply by 100 (3/4 → 0.75 → 75%). To go from a percent to a decimal, divide by 100 (75% → 0.75); to a fraction, write it over 100 and simplify (75/100 → 3/4).
Find the part means the percent and whole are known and the piece is missing (what is 20% of 50?); find the percent means the part and whole are known and the relationship between them is missing (15 is what percent of 50?); find the whole means the part and percent are known and the total is missing (20 is 40% of what number?).
Divide the amount of change by the original amount, then convert to a percent: a price that goes from $40 to $50 changed by $10, and 10 ÷ 40 = 0.25, or 25%. Find the Percent Change of a Quantity always divides by the original value, not the new one.
The calculation is identical — divide the change by the original amount — the only difference is whether the quantity went up (increase) or down (decrease), which is easy to mix up if a child isn’t sure which number is the ‘original’ one to divide by.
Multiply the original price by the discount percent to find the amount taken off, then subtract that from the original price — a $50 item at 20% off has a discount of 50 × 0.20 = $10, so the final price is $40. Solve Percent Problems Involving Discounts, Markups, and Tips also covers the shortcut of multiplying directly by 80% (100% − 20%) to get the final price in one step.