NUMBER

Prime Numbers

Learn the reliable method for prime numbers, then apply it to 11+ exam-style questions.

1 · WHAT YOU NEED TO KNOW

The idea

Prime Numbers uses place value and number relationships to compare, represent and reason with numbers.

Picture itPick out numbers with two factors
2 · THE METHOD

Follow these steps

  1. Identify the value of each digit or number.
  2. Apply the requested rule carefully.
  3. Check the result on a number line or with an inverse where possible.
3 · WORKED EXAMPLES

See the method in action

Example 1

I am a two-digit prime between 70 and 80. My digits total 8, and reversing my digits also makes a prime. Which number am I?

  1. Check the interval and digit sum first.
  2. 71 has digit sum 8; it is prime, and its reversal 17 is also prime.
Answer: 71
Example 2

I am a two-digit prime between 70 and 80. My digits total 8, and reversing my digits also makes a prime. Which number am I?

  1. Check the interval and digit sum first.
  2. 71 has digit sum 8; it is prime, and its reversal 17 is also prime.
Answer: 71
4 · ELEVENSPRINT TIP

A useful shortcut

Underline the place value or condition the question asks about.

5 · COMMON MISTAKES

Watch out for these

Looking at the wrong neighbouring digit

For rounding, inspect the digit immediately to the right of the rounding place.

6 · TRY IT YOURSELF

Three fresh questions

Ready to check the method? You will get three fresh, validated questions.