Prime factorisation for the 11 plus

Split the number into any two factors, then keep splitting each one until every branch ends in a prime. The ends of the branches are the answer. It does not matter how you start. The recipe comes out the same.

320 is 32 times 10. Thirty-two splits into two and sixteen, sixteen into two and eight, eight into two and four, four into two and two. That is five twos. Ten is two times five. So altogether: six twos and a five.

The trap is stopping at a four. Four times two times two times two times two times five has the right product, but four is not prime. It is not finished until every branch is a prime.

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
• 1.6Prime numbers, factors and multiplesShared factors make it smaller
• 1.7Square and cube numbersA root is not a half

Every 11 plus maths topic