learnbits.app· Geography

Sums with zeros and thought clouds

You learn to use a small sum to help you work out a bigger one.

Practise thisWhiteboardMake it a quiz

How to work it out

  1. Leave the zeros out for a moment: 4 × 6 = 24.
  2. Then put the two zeros back: 4 × 600 = 2400.
Denk aan:
4 × 6 = 24

A worked example

4 × 7 = ? when you know 4 × 6 = 24

  1. 4 × 6 = 24.
  2. You need one more group of 4.
  3. 24 + 4 = 28.
  4. So 4 × 7 = 28.

Try it yourself

  1. Denk aan:
    15 ÷ 3 = 5

    1500 ÷ 30 =

  2. Denk aan:
    24 ÷ 4 = 6

    240 ÷ 4 =

Watch out

  • You put the zeros back wrongly: 4 × 600 becomes 240 or 24,000.

    First count how many zeros you leave off. After the helper sum, put exactly that many back.

  • You forget the zeros that are already in the helper sum: 5 × 6 = 30 already has a zero.

    That zero belongs to the answer of the helper sum. The zeros you left off still go after it.

Tips

  • Use what you know to work out what you don't know yet.

Why it works

If you know 5 × 3 = 15, you can work out 5 × 4 without counting all four fives. You just add one more: 15 + 5 = 20.