Cogito
Grade 5 Math · Chapter 5 · Lesson 1
Finding a Common Denominator
Cut both into the same size pieces.
10 problems · about 15 minutes · 5.NF.A.1
What this lesson teaches
The child finds a common denominator for two fractions and rewrites both.
- Fractions can only be counted together in one shared unit.
- Multiplying the two denominators always gives one.
- Both numbers of each fraction change together.
Warm-Up Review
From earlier lessons. Loosen up before the new work.
2 problemsReview — Dividing Decimals: How many halves fit into 9?
Answer 18
Why 18 halves.
Review — When Multiplying Makes Things Smaller: What is 0.25 × 40?
Answer 10
Why 10.
Warm Up
Straightforward practice. Get the method working first.
4 problems2/3 written in fifteenths. What is the top number?
Answer 10
Why 10/15.
What method for finding a common denominator always works?
Answer Multiply the two bottom numbers.
Why Multiplying them always gives a number both divide into.
A common denominator for halves and fifths?
Answer 10
Why 2 × 5.
A common denominator for thirds and sixths?
Answer 6
Why 6 already works.
Build It Up
The same ideas with more to keep track of.
2 problems1/2 written in tenths. What is the top number?
Answer 5
Why 2 × 5 = 10.
3/5 written in tenths. What is the top number?
Answer 6
Why Both numbers double.
Stretch Yourself
Mixed problems. Work out what kind of question it is before you start.
2 problemsThe Shared Unit: 2/3 written in twelfths. What is the top number?
Answer 8
Why 8/12.
The Other One: 3/4 written in twelfths. What is the top number?
Answer 9
Why 9/12.