Maths Unit 1 (F) - Number Multiples Primes A prime number is a number which only has two factors – itself & 1. The first 10 prime numbers are: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29 A common mistake is that people think 1 is a prime number. The only way to make 1 is by doing 1 x 1 , so the number 1 only has one factor NOT two factors, therefore it is NOT prime. Examples: Is 11 prime? - 1 x 11 Yes it’s prime Is 12 prime? – 1 x 12, 2 x 6, 3 x 4 No it’s not prime A multiple is a number which can be divided by another number without a remainder. For example: 15, 20 100, 150 are multiples of 5 because they can be divided by 5 and have no remainder Factors An exact divisor of a number is called a factor. Estimating When estimating an answer. Always round numbers to one significant figure (sig fig) and then do the calculation. Factors can be written in pairs to help find all the factors of a given number. E.g. Factors of 12 can be written like this….. Prime Factorisation All numbers can be written as a product of their primes i.e. each number can be made by multiply prime numbers together. A factor tree helps you find which prime numbers multiply to make a given number. Key words and definitions Multiple - a number which can be divided by another number without a remainder. Divisor – the number you divide by. Factor - An exact divisor of a number. Prime - a number which only has two factors Product – the result of multiplying to numbers together Prime Factorisation – a list of prime numbers which multiply together to create another number E.g. The prime factors of 75 are 3, 5 and 5. It is said that 75 is a product of 3 x 5 2 .