Cogito
Grade 7 Math · Chapter 3 · Lesson 3
Discounts and Sale Prices
Taking a percent off.
10 problems · about 18 minutes · 7.RP.A.3
What this lesson teaches
The child finds a sale price after a percent discount.
- A discount is easier as what is left than as what is taken.
- Taking p% off means multiplying by 1 − p/100.
- Two discounts multiply; they never simply add.
Warm-Up Review
From earlier lessons. Loosen up before the new work.
3 problemsReview — Tax, Tips, and Markup: £90 with 20% added. Total in pounds?
Answer 108
Why £108.
Review — Percent Increase and Decrease: A 120 pound item falls 25%. New price?
Answer 90
Why 90 pounds.
Review — Unit Rates with Fractions: ⅖ mile in ⅕ hour. Miles per hour?
Answer 2
Why 2 mph.
Warm Up
Straightforward practice. Get the method working first.
3 problems£70 with 20% off. Pay in pounds?
Answer 56
Why £56.
£40 with 10% off. Pay in pounds?
Answer 36
Why × 0.9.
£60 with 50% off. Pay in pounds?
Answer 30
Why Half.
Build It Up
The same ideas with more to keep track of.
2 problemsWhat multiplier takes 40% off?
Answer 0.6
Why What is left?
£100, 10% off then 10% off. Pay in pounds?
Answer 81
Why 0.9 × 0.9.
Stretch Yourself
Mixed problems. Work out what kind of question it is before you start.
2 problemsThe Sale: £80 with 25% off. What do you pay, in pounds?
Answer 60
Why £60.
The Second Discount: £200, 50% off then 50% off again. What do you pay, in pounds?
Answer 50
Why £50 — not free.