Prime factorisation

Writing numbers as a product of primes, and finding the HCF and LCM. Read one part at a time and tap “I know this”. Then test yourself in the quiz.

Your progress0 of 2

Write a number as a product of primes. Use a factor tree or keep dividing by the smallest prime.

Rule

72 = 2 × 36 = 2 × 2 × 18 = 2 × 2 × 2 × 9 = 2 × 2 × 2 × 3 × 3 = 2³ × 3²

Tap when you could tell someone without looking.

HCF and LCM with prime factors

HCF: multiply only the prime factors that appear in both, each the smallest number of times it appears in either number.
LCM: multiply all the factors, each the greatest number of times it appears.

Rule

12 = 2² × 3 and 18 = 2 × 3² → HCF = 2 × 3 = 6; LCM = 2² × 3² = 36.

Another one: 12 = 2² × 3 and 24 = 2³ × 3 → HCF = 2² × 3 = 12 (the 2 only twice); LCM = 2³ × 3 = 24.

Tap when you could tell someone without looking.

Remember this: your summary

Write these points in your book. Read them again the night before the test.

Memory trick

Chop the tree down to the prime leaves.

Flash cards · Tap to turn over

First say the answer yourself, then turn the card over.

Test yourself · Part 2

Read the question and the answer. Is the answer right? Tap True or False.

Now the real quiz!

The quiz has many more questions on this topic. Every question shows straight away whether you are right and explains why.

For parents

Let your child work the examples on paper and explain them to you.
The ticks (“I know this”) are saved on this device only. We never ask for names or emails.

Follows CAPS for Grade 7 Mathematics. Always check your teacher’s notes and textbook too.