How to Learn the Prime Numbers
A prime number has exactly two factors — 1 and itself. Knowing the primes up to 50 by heart (and the tricks to test any number) speeds up fractions, factoring and number theory. This deck drills the primes and the shortcuts.
There are 12 cards in the deck below. You can read through them, practise them right here with the flip-card player, and then save the deck to study properly with spaced repetition — the method that makes them stick for months, not minutes.
Why the prime numbers is hard to memorize (and the fix)
The tricky part is the "is it prime?" judgement — 51 looks prime but is 3×17; 91 looks prime but is 7×13. A few divisibility rules and some practice make it instant.
The fix is active recall plus spaced repetition. Instead of re-reading the list (which feels productive but barely works), you test yourself — pull each answer out of memory — and you review each card on a schedule that stretches over time. Every time you successfully recall a fact, the memory gets stronger and the next review is pushed further out. That's how you go from "I saw it once" to "I just know it."
The fastest way to learn the prime numbers
- Break it into small batches. Memorize the 15 primes under 50 as a set, then practise the "trap" composites that look prime (51, 57, 91).
- Test, don't re-read. Look at the question, say the answer out loud before flipping the card. The little struggle to recall is what builds the memory — a smooth re-read does almost nothing.
- Review on a schedule. Spaced repetition shows you each card right before you'd forget it. Get one right and it comes back in a few days, then a week, then a month. Miss one and it comes back sooner.
- Keep sessions short and daily. Five to ten minutes a day beats a two-hour cram every time. Consistency is the whole game.
One fact per card. Learn the divisibility rules (by 2, 3, 5) — they rule out most non-primes in seconds.
The full deck: the Prime Numbers
Here's every card in the set. Practise them above, or save the deck and let spaced repetition schedule your reviews automatically.
| Question | Answer |
|---|---|
| What is a prime number? | A whole number greater than 1 with exactly two factors: 1 and itself |
| What is the only even prime number? | 2 |
| What is the smallest prime number? | 2 |
| List the prime numbers under 20. | 2, 3, 5, 7, 11, 13, 17, 19 |
| List the prime numbers between 20 and 50. | 23, 29, 31, 37, 41, 43, 47 |
| Is 1 a prime number? | No — 1 has only one factor, so it is neither prime nor composite |
| Is 51 prime? | No — 51 = 3 × 17 |
| Is 57 prime? | No — 57 = 3 × 19 |
| Is 91 prime? | No — 91 = 7 × 13 |
| Is 97 prime? | Yes |
| How many prime numbers are there below 100? | 25 |
| What is the largest prime under 50? | 47 |
Save this deck and actually remember it
Create a free account, keep this 12-card deck, and we'll schedule every review at the perfect moment. First deck free forever — no credit card.
Study this deck free →FAQ: learning the prime numbers
How long does it take to learn the prime numbers?
Most people can lock in the prime numbers within one to three weeks of short daily sessions. With spaced repetition doing 10–12 cards a day, you'll recognise them within days and recall them reliably within a couple of weeks.
What's the best way to memorize the prime numbers?
Active recall plus spaced repetition. Test yourself instead of re-reading, and review on a schedule that spaces the cards out over time. Flashcards are built for exactly this — which is why the deck above is the fastest path.
Is this deck free?
Yes. You can practise all 12 cards on this page for free with no account. Sign up (also free) to save the deck and let spaced repetition schedule your reviews so it sticks long-term.