Cogito
Grade 4 Math · Chapter 4 · Lesson 2
Prime and Composite
Numbers that make only one rectangle.
10 problems · about 14 minutes · 4.OA.B.4
What this lesson teaches
The child classifies a whole number to 100 as prime or composite.
- A prime number has exactly two factors.
- A composite number has more than two.
- 1 has only one factor, so it is neither.
Warm-Up Review
From earlier lessons. Loosen up before the new work.
2 problemsReview — Factor Pairs: How many factors does 36 have?
Answer 9
Why 36 has 9 factors, because 6 × 6 counts only once.
Review — Multistep Word Problems: 7 × 24 − 68 = ?
Answer 100
Why 7 × 24 = 168, and 168 − 68 = 100.
Warm Up
Straightforward practice. Get the method working first.
4 problemsIs 29 prime?
Answer Yes.
Why 29 has only 1 and 29 as factors.
Why is 1 not a prime number?
Answer It has only one factor, not two.
Why 1 and itself are the same number here, so there is only one factor.
Is 17 prime?
Answer Yes.
Why Nothing from 2 to 4 divides it.
Is 27 prime?
Answer No. 3 × 9 = 27.
Why Try 3.
Build It Up
The same ideas with more to keep track of.
2 problemsHow many factors does a prime number have?
Answer 2
Why 1 and itself.
What is the only even prime number?
Answer 2
Why Every other even number has 2 as a factor.
Stretch Yourself
Mixed problems. Work out what kind of question it is before you start.
2 problemsThe Class: 13 children want equal rows of more than one. Is it possible?
Answer No. 13 is prime.
Why A prime number of children cannot make equal rows.
The Other Class: 15 children in equal rows of more than one. How many in each row, if there are 3 rows?
Answer 5 children
Why 15 is composite, so this works: 5 in each row.