Which of these numbers is a prime number?
Show worked explanation
A prime number only has two factors: 1 and itself. 17 can only be divided by 1 and 17, so it's prime. The others have more factors. ✓
11+ Prime Numbers & Factors Practice
(GL Assessment)
Prime numbers, factors and multiples make up two to four questions in every GL Assessment 11+ maths paper, scattered across the difficulty range and drawn from the Year 5 and Year 6 curriculum. It is one of the more predictable topics on the paper: the question formats repeat year after year, so a well-prepared child can reliably bank these marks.
The topic covers a connected family of ideas: spotting prime numbers up to 100, listing factors and factor pairs, recognising multiples, and finding the highest common factor (HCF) and lowest common multiple (LCM) of two numbers. GL also likes square and cube numbers, and the occasional factor tree.
Some of this stretches just beyond what schools teach. HCF, LCM and prime factorisation are not always covered in depth at primary level, yet they appear in GL papers, so a little targeted practice goes a long way. The good news for an anxious parent: once your child knows their times tables and the primes to 100, most of these questions become quick, confident wins.
What the GL 11+ Tests on Prime Numbers & Factors
All questions are five-option multiple choice (A to E). Based on our analysis of GL papers and leading tutor resources, the sub-skills appear in roughly this order of frequency (weightings are our research estimates, not published by GL):
- Finding factors (around 20%): listing factor pairs, counting how many factors a number has.
- Identifying primes (around 15%): recognising primes to 100, knowing 1 is not prime and 2 is the only even prime.
- Common factors and HCF (around 15%).
- Multiples (around 15%): including telling multiples and factors apart.
- Square and cube numbers (around 10%).
- Common multiples and LCM (around 10%).
- Prime factorisation / factor trees (around 10%).
- Divisibility rules (around 5%).
Difficulty runs from simple "which of these is prime?" recall (D1) through HCF and LCM in word problems (D2) up to multi-step reasoning such as "a number has exactly 3 factors" (D3). HCF, LCM and formal prime factorisation are the areas most likely to sit beyond your child's school coverage.
Sample Prime Numbers & Factors Questions
Five questions drawn from PrepStep’s prime numbers & factors bank, spanning Foundation to Challenging. Tap “Show worked explanation” to see the full method after you’ve had a go. The correct answer is highlighted on each question so you can check immediately.
A maths challenge card asks: how many factors does 24 have?
Show worked explanation
The factors of 24 are: 1, 2, 3, 4, 6, 8, 12, 24. Count them: that's 8 factors. ✓
Emma wants to arrange 18 chairs in equal rows with no chairs left over. Which of these is NOT a possible number of rows?
Show worked explanation
The number of rows must be a factor of 18. Factors of 18 are: 1, 2, 3, 6, 9, 18. The number 4 is NOT a factor of 18. ✓
Which number is both a factor of 30 and a factor of 45?
Show worked explanation
Factors of 30: 1, 2, 3, 5, 6, 10, 15, 30. Factors of 45: 1, 3, 5, 9, 15, 45. Common factors include 1, 3, 5, and 15. From the options, 15 is a factor of both. ✓
In a quiz, the question says: which number below has exactly 3 factors?
Show worked explanation
A number with exactly 3 factors must be the square of a prime. 9 = 3² and has factors: 1, 3, 9. That's exactly 3 factors. ✓
Common Mistakes to Avoid
Common mistake 1 of 4
Thinking 1 is prime, or that 2 is not.
Tip: 1 is not prime (it has only one factor); 2 is prime and is the only even prime. A prime has exactly two factors, no more and no fewer.
Common mistake 2 of 4
Believing all odd numbers are prime.
Tip: 9, 15, 21, 25, 51 and 91 are all odd but not prime. Memorise the "looks prime but isn't" list, especially 51 (3 x 17) and 91 (7 x 13), which GL uses on purpose.
Common mistake 3 of 4
Mixing up factors and multiples.
Tip: Factors fit inside a number; multiples grow out of it. Factors of 12 are small (1, 2, 3, 4, 6, 12); multiples of 12 are large (24, 36, 48).
Common mistake 4 of 4
Swapping HCF and LCM.
Tip: Highest Common Factor is the biggest shared factor; Lowest Common Multiple is the smallest shared multiple. Do not just multiply the two numbers for the LCM, as that only works when they share no common factor.
Frequently Asked Questions
What prime and factor topics are on the GL 11+ maths exam?
GL tests identifying primes up to 100, listing factors and factor pairs, finding multiples, highest common factor (HCF), lowest common multiple (LCM), prime factorisation, and square and cube numbers. These make up two to four questions per paper.
Is 1 a prime number for the 11+?
No. 1 is not prime because a prime number must have exactly two factors, and 1 has only one. The smallest prime number is 2, which is also the only even prime. GL tests this point deliberately.
Which numbers look prime but aren't?
The classic traps are 51 (3 x 17), 57 (3 x 19), 87 (3 x 29) and 91 (7 x 13). All are odd and not in the common times tables, so children assume they are prime. GL uses them as distractors regularly.
What's the difference between HCF and LCM?
HCF, the highest common factor, is the largest number that divides into two numbers (HCF of 12 and 18 is 6). LCM, the lowest common multiple, is the smallest number both divide into (LCM of 12 and 18 is 36). Children often confuse the two.
Does the 11+ test things schools don't teach?
Somewhat. The curriculum covers primes, factors and multiples, but HCF, LCM and formal prime factorisation with factor trees are pushed harder in GL papers than in many primary classrooms. A few weeks of focused practice on these closes the gap.
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