https://www.khanacademy.org/math/cc-fourth-grade-math/imp-factors-multiples-and-patterns/imp-prime-and-composite-numbers/v/prime-numbers 2021-06-21T15:29:12Z Prime numbers Prime numbers are numbers that have only 2 factors: 1 and themselves. For example, the first 5 prime numbers are 2, 3, 5, 7, and 11. By contrast, numbers with more than 2 factors are call composite numbers. Sal Khan video https://cdn.kastatic.org/googleusercontent/sybkK0hAAkEE-Ubw97XuaaOw80eEcS9b8X9i8GmF5nQcz2rGJcxEJvcACkMi6bzEiYhJbkaaklYEhPaKU79McX-3 Prime numbers Prime numbers are numbers that have only 2 factors: 1 and themselves. For example, the first 5 prime numbers are 2, 3, 5, 7, and 11. By contrast, numbers with more than 2 factors are call composite numbers. https://cdn.kastatic.org/ka-youtube-converted/mIStB5X4U8M.mp4/mIStB5X4U8M.mp4 493 Illustrating prime and composite integers https://www.khanacademy.org/math/cc-fourth-grade-math/imp-factors-multiples-and-patterns/imp-prime-and-composite-numbers/v/recognizing-prime-numbers 2021-06-21T15:29:05Z Recognizing prime and composite numbers Can you recognize the prime numbers in this group of numbers? Which are prime, composite, or neither? Sal Khan video https://cdn.kastatic.org/googleusercontent/3CUVy3dSECgSfUGKi3RzIaOtOmHZYUAof3q6vaMavHwEy58o3myPKm7bNTBW4qJi9ZJ_siacCvXZcAlrcbqtMlzx Recognizing prime and composite numbers Can you recognize the prime numbers in this group of numbers? Which are prime, composite, or neither? https://cdn.kastatic.org/ka-youtube-converted/3h4UK62Qrbo.mp4/3h4UK62Qrbo.mp4 280 Illustrating prime and composite integers https://www.khanacademy.org/math/class-6-tg/x47164056520f4c0a:playing-with-numbers/x47164056520f4c0a:prime-factorisation/v/methods-of-prime-factorisation 2026-01-16T09:40:42.440263577Z Methods of Prime Factorisation Prime factorisation is all about breaking numbers down into their simplest prime building blocks, and in this video we explain two easy and effective methods to do it: the division method and the factor tree method. Using clear step-by-step examples, you’ll see how the division method works by repeatedly dividing a number by the smallest prime until you reach 1, while the factor tree method splits the number into factors and keeps breaking them down until only primes remain. Both approaches lead to the same unique prime factorization, showing the consistency and beauty of number theory. Khan Academy video https://cdn.kastatic.org/googleusercontent/ZCdwTudJg6e6n-P2gsaUborP4izvMsGo71pvEVlX9dNYWcLXcP7VHkWpn2grt4TUP1KoJLQP9NswyHBuBLSFTBw Methods of Prime Factorisation Prime factorisation is all about breaking numbers down into their simplest prime building blocks, and in this video we explain two easy and effective methods to do it: the division method and the factor tree method. Using clear step-by-step examples, you’ll see how the division method works by repeatedly dividing a number by the smallest prime until you reach 1, while the factor tree method splits the number into factors and keeps breaking them down until only primes remain. Both approaches lead to the same unique prime factorization, showing the consistency and beauty of number theory. https://cdn.kastatic.org/ka-youtube-converted/IqdOpbEZfnI.mp4/IqdOpbEZfnI.mp4 368 Illustrating prime and composite integers https://www.khanacademy.org/computing/computer-science/cryptography/random-algorithms-probability/v/fermat-primality-test-prime-adventure-part-10 2021-03-22T14:52:39Z Fermat primality test A quick outline of how & why it works. Brit Cruise video https://cdn.kastatic.org/googleusercontent/6R-1y-H1XzM_GVjsjK0smoIROBTNS4Sn3gxwpAuU_g7Ojc-glKdFpFaoy8d6vFwBbdkyS5YcVmOjsuQfR0D2w7dN-w Fermat primality test A quick outline of how & why it works. https://cdn.kastatic.org/ka-youtube-converted/oUMotDWVLpw.mp4/oUMotDWVLpw.mp4 429 Illustrating prime and composite integers https://www.khanacademy.org/math/class-6-ncf/x04ca845a9a148c80:prime-time/x04ca845a9a148c80:co-prime-numbers/v/co-prime-and-twin-prime-numbers 2026-01-16T09:40:42.440263577Z Co-Prime and Twin prime Numbers In this video, we explore two important concepts in number theory: co-prime numbers and twin primes. Co-prime numbers are pairs of integers that share no common factor other than 1, and we also discuss their special divisibility property—if two co-primes both divide a number, then their product also divides that number. Alongside this, we introduce twin primes, which are pairs of prime numbers differing by exactly 2, such as (11, 13) or (17, 19). With clear explanations and examples, this video makes these ideas simple to understand and shows how they reveal unique relationships between numbers. Khan Academy video https://cdn.kastatic.org/googleusercontent/ZCdwTudJg6e6n-P2gsaUborP4izvMsGo71pvEVlX9dNYWcLXcP7VHkWpn2grt4TUP1KoJLQP9NswyHBuBLSFTBw Co-Prime and Twin prime Numbers In this video, we explore two important concepts in number theory: co-prime numbers and twin primes. Co-prime numbers are pairs of integers that share no common factor other than 1, and we also discuss their special divisibility property—if two co-primes both divide a number, then their product also divides that number. Alongside this, we introduce twin primes, which are pairs of prime numbers differing by exactly 2, such as (11, 13) or (17, 19). With clear explanations and examples, this video makes these ideas simple to understand and shows how they reveal unique relationships between numbers. https://cdn.kastatic.org/ka-youtube-converted/JTqQtJYIG04.mp4/JTqQtJYIG04.mp4 269 Illustrating prime and composite integers https://www.khanacademy.org/computing/computer-science/cryptography/modern-crypt/v/euler-s-totient-function-phi-function 2021-07-08T12:13:20Z Euler's totient function Measuring the divisibility of a number Brit Cruise video https://cdn.kastatic.org/googleusercontent/bO5pu-2tZ1uLy6EGe1v5LI7_IdeQfc-qpeokNcaxtDNSWrN7sPhadMW_KjGw3mkwHbzAldF3NrGMvqHDtk5H20NXfQ Euler's totient function Measuring the divisibility of a number https://cdn.kastatic.org/ka-youtube-converted/qa_hksAzpSg.mp4/qa_hksAzpSg.mp4 138 Illustrating prime and composite integers https://www.khanacademy.org/math/cc-fourth-grade-math/imp-factors-multiples-and-patterns/imp-prime-and-composite-numbers/a/prime-and-composite-review 2022-05-23T14:30:02.465161589Z Prime and composite numbers review Review prime and composite numbers, and try some practice problems. article https://www.khanacademy.org/math/cc-fourth-grade-math/imp-factors-multiples-and-patterns/imp-prime-and-composite-numbers/e/prime_numbers 2025-01-31T11:23:03.723679407Z Identify prime numbers Identify prime numbers less than 100. Marcia Lee exercise https://www.khanacademy.org/math/cc-fourth-grade-math/imp-factors-multiples-and-patterns/imp-prime-and-composite-numbers/e/composite_numbers 2025-01-31T11:23:03.723679407Z Identify composite numbers Identify composite numbers less than 100. Ben Alpert exercise https://www.khanacademy.org/math/cc-fourth-grade-math/imp-factors-multiples-and-patterns/imp-prime-and-composite-numbers/e/prime-and-composite-numbers 2026-04-07T23:55:07.30621384Z Prime and composite numbers Practice identifying prime and composite numbers. Lindsay exercise https://www.khanacademy.org/math/class-6-ncf/x04ca845a9a148c80:prime-time/x04ca845a9a148c80:co-prime-numbers/e/co-prime-and-twin-prime-numbers 2026-01-16T09:40:42.440263577Z Co-Prime and Twin prime Numbers In this exercise, we will learn about (a) Two numbers with only 1 as a common factor are called co-prime numbers. (b) Twin primes are prime numbers that differ from each other by two e.g. (3, 5), (5, 7) (c) A number divisible by two co-prime numbers is also divisible by their product. Khan Academy exercise