Cogito
Grade 6 Math · Chapter 2 · Lesson 4
Comparing with Unit Rates
Which one is the better buy?
10 problems · about 18 minutes · 6.RP.A.2, 6.RP.A.3
What this lesson teaches
The child compares two options by working out and comparing their unit rates.
- Two different pack sizes cannot be compared by price alone.
- Divide to find the cost of exactly one of each.
- The smaller cost per one is the better buy.
Warm-Up Review
From earlier lessons. Loosen up before the new work.
3 problemsReview — Finding a Percent of a Number: What is 30% of 90?
Answer 27
Why 3 × 9 = 27.
Review — Percent as a Rate per Hundred: 17 out of 20 as a percent?
Answer 85
Why 85%.
Review — Unit Rates: 84 miles in 2 hours. How many miles per hour?
Answer 42
Why 42 miles per hour.
Warm Up
Straightforward practice. Get the method working first.
3 problems6 cakes for £9. Price for one, in pounds?
Answer 1.5
Why £1.50 each.
3 kg for £9. Price per kg, in pounds?
Answer 3
Why 9 ÷ 3.
4 kg for £10. Price per kg, in pounds?
Answer 2.5
Why 10 ÷ 4.
Build It Up
The same ideas with more to keep track of.
2 problemsOf those two, which is the better buy?
Answer The 4 kg bag.
Why £2.50 against £3.
200 g for £4. Price per 100 g, in pounds?
Answer 2
Why Half the weight, half the price.
Stretch Yourself
Mixed problems. Work out what kind of question it is before you start.
2 problemsThe Juice Aisle: 5 bottles cost £10. What is the price of one bottle, in pounds?
Answer 2
Why £2 each.
The Other Shelf: 8 bottles cost £12. What is the price of one bottle, in pounds?
Answer 1.5
Why £1.50 each, so this shelf is cheaper.