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5th grade · Math · Unit 15 of 15

Prime Factor Trees

Break numbers into primes

What your child practices

Writing whole numbers from 2 to 50 as products of prime factors, for example 24 = 2 × 2 × 2 × 3, using factor trees. Prime factorization is a California addition to the 5th grade standards.

California Content Standards learning standard: 5.OA.2.1 · expressing a whole number from 2 to 50 as a product of its prime factors

How we explain it

Children can open this short lesson before they practice.

  1. A prime number has exactly two factors: 1 and itself (2, 3, 5, 7, 11 …).
  2. Make a factor tree: split the number into two factors, then keep splitting any factor that isn't prime.
  3. The primes at the ends of the branches, multiplied together, make the number.

Example: Write 24 as a product of prime factors. 24 = 2 × 12. 12 = 2 × 6, and 6 = 2 × 3. 24 = 2 × 2 × 2 × 3. Answer: 2 × 2 × 2 × 3

Sample questions

A taste of this unit. In the app, questions change every time and get easier or harder to match your child.

  1. Question 1

    Which number is prime?

    • 42
    • 38
    • 28
    • 41

  2. Question 2

    In a factor tree, 24 splits into 2 × 12. What prime factors does 12 split into?

    • 1 × 12
    • 2 × 2 × 3
    • 2 × 2 × 2 × 3
    • 2 × 12

  3. Question 3

    How many prime factors does 10 have, counting repeats?

    • 2
    • 5
    • 4
    • 3

  4. Question 4

    Write 30 as a product of prime factors.

    • 5 × 6
    • 2 × 15
    • 2 × 3 × 5
    • 3 × 10

  5. Question 5

    24 = ? × 2 × 2 × 3. What prime number is missing?

    • 3
    • 5
    • 4
    • 2

  6. Question 6

    Which number has the prime factorization 3 × 5?

    • 6
    • 15
    • 24
    • 18

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