How to play Prime Time

  1. Choose a level, then press Start game.
  2. A number comes into the detector. Tap Prime or Not prime.
  3. The dots show why: a number that is not prime makes a rectangle.
  4. On Factor pairs, pick the two numbers that multiply to make the number.
  5. On Divisibility, decide whether the number divides exactly, then read the rule.

For parents and teachers

A prime number has exactly two factors, 1 and itself, and the dot picture makes that concrete: 15 dots make a 3 by 5 rectangle, but 13 dots only make a line. Ask your child to try making rectangles with buttons or coins at home. The divisibility rules, such as adding the digits to test for 3, are worth learning by heart because they make checking big numbers quick.