Prime numbers · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Prime numbers

Mathematics · Multiplication & Division · ages 9-10
Name ______________________   Date ____________

  1. 1 is a prime number.

    Circle one:   True   False

  2. True or False: 2 is a prime number.

    Circle one:   True   False

  3. Which of these numbers is prime?

    • 15
    • 21
    • 17
    • 27

  4. A prime number has exactly two factors. What are they?

    • 1 and itself
    • 2 and itself
    • Any two numbers

  5. 51 is a composite number, not a prime number.

    Circle one:   True   False

  6. How many factors does 59 have?

    Answer: ______________

  7. Is 29 prime or composite?

    • Prime
    • Composite

  8. True or False: The number 1 is a prime number.

    Circle one:   True   False

  9. Leo tests 51 by dividing only by 2, 5, and 7, finds none work exactly, and says 51 is prime. What did Leo get wrong?

    • He forgot to try dividing by 3
    • 51 is too big to be prime
    • He should have divided by 10

  10. Which group contains only prime numbers up to 19?

    • 4, 7, 13, 19
    • 2, 3, 9, 11
    • 2, 5, 11, 17
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Prime numbers W1-mt_y1XCVsIelg-s1

  1. False · 1 has only one factor, itself. A prime number needs exactly two factors.
  2. True · 2 can only be divided evenly by 1 and 2, so it counts as prime, even though it's even.
  3. 17 · 17 can only be divided evenly by 1 and 17.
  4. 1 and itself · Every prime number can only be divided exactly by 1 and by itself, nothing else.
  5. True · 51 = 3 x 17, so it has more than two factors, which makes it composite.
  6. 2 · 59 is prime, so it has exactly two factors: 1 and 59.
  7. Prime · No number between 2 and 28 divides evenly into 29, so its only factors are 1 and 29.
  8. False · 1 only has one factor, itself, so it doesn't meet the rule of having exactly two factors.
  9. He forgot to try dividing by 3 · 51 divided by 3 is 17 exactly, so Leo needed to check 3 too. Skipping it made him miss that 51 is composite.
  10. 2, 5, 11, 17 · 2, 5, 11, and 17 each only divide by 1 and themselves. The other groups hide a composite number, 9 or 4.
Worksheet · LightMySky