Classifying Numbers by Their Properties
Primary 4 · Mathematics
Classifying Numbers by Their Properties
Lesson 3 of 5
25 min
Let's begin
Gatete tries to arrange 7 chairs in rows for a meeting, with the same number in every row. He can only make one long row of 7! With 6 chairs he can make 2 rows of 3, or 3 rows of 2. Why is 7 so stubborn?
A prime number has exactly two factors: 1 and itself.
A composite number has more than two factors. 6 is composite: its factors are 1, 2, 3 and 6.
1 has only one factor, itself. So 1 is neither prime nor composite.
There are 15 prime numbers up to 50. To test a number up to 100, try dividing it by 2, 3, 5 and 7. If one of them divides it exactly (and it is not that number itself), the number is composite.
Watch out! Not every odd number is prime: 9 = 3 × 3, 15 = 3 × 5, 21 = 3 × 7 and 49 = 7 × 7 are all odd but composite.
Tip
Prime means "only two". A prime number has only two factors, 1 and itself, just like only one row of chairs fits it.
Work each one in your exercise book first. Then tap Show answer to check.
Is 13 prime or composite?
Answer: Prime. Its only factors are 1 and 13.
Is 51 prime or composite?
Answer: Composite, because 51 = 3 × 17.
Write the prime numbers between 10 and 20.
Answer: 11, 13, 17, 19
Which even number is prime?
Answer: 2
Umutoni says 1 is a prime number. Is she right?
Answer: No. 1 has only one factor, so it is neither prime nor composite.
What you learned today
Sources and credits
The stories, Rwandan worked examples, diagrams and practice questions were written by Ganzaa.
Quick check
Which number is prime?
