Exercises: Prime Factorisation

Work through each section in order. Write every prime factorisation as a product of primes, using index (exponent) notation where a prime repeats. Verify larger factorisations by multiplying the primes back to the original number.

Grade 7·16 problems·~28 min·Uganda NCDC Primary 7 Mathematics·lesson·operations-04

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Recall / Warm-Up

These problems review prime and composite numbers from earlier work.

Which of these numbers is prime?

$21$

$23$

$27$

$1$

How many prime numbers are there between $1$ and $20$ (not counting $1$ )?

Fluency Practice

Which number below is composite?

$17$

$29$

$33$

$41$

$Factor tree for 90 branching into 9 and 10, then into prime leaves 3, 3, 2, and 5$

Use the factor tree to find the prime factorisation of $90$ . Write your answer as a product of primes using index notation.

Find the prime factorisation of $84$ using a factor tree. Write your answer as a product of primes using index notation.

$Division ladder for 378 dividing successively by 2, 3, 3, 3, and 7 down to a quotient of 1$

Use the division ladder to find the prime factorisation of $378$ . Write your answer as a product of primes using index notation.

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