Prime numbers, factors and multiples for the 11 plus

A factor divides a number exactly. The factors of twelve come in pairs: one and twelve, two and six, three and four. A multiple is the number times something, so twelve, twenty-four, thirty-six. A prime has exactly two factors, one and itself. Two, three, five, seven, eleven. One is not prime, it has only one factor.

The lowest common multiple of 86 and 129. Look closely. Eighty-six is two lots of forty-three. One twenty-nine is three lots of forty-three. They share forty-three. So the lowest number both reach is forty-three times two times three. 258.

The trap is just multiplying them: 11,094. It is a common multiple, but nowhere near the lowest. When two numbers share a factor, you can use it once.

Part of the Number & Place Value topics for the 11+.

• 1.1Reading and writing large numbersOne zero can change everything
• 1.2Rounding numbersLook one digit to the right
• 1.3Comparing and orderingThe first difference decides it
• 1.4Roman numeralsSmall before big means take away
• 1.5Negative numbersCrossing zero? Count both sides
• 1.7Square and cube numbersA root is not a half
• 1.8Prime factorisationSplit until every end is prime

Every 11 plus maths topic