learnbits.app· Geography

Subtraction within twenty

You learn subtractions to twenty where the ten stays and you only take away from the ones.

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How to work it out

  1. Keep the ten.
  2. Take the ones away from the ones.
  3. If a 10 goes too, take it away from the ten.

A worked example

18 − 5

  1. Leave the 10 in 18 alone.
  2. Take away from the ones: 8 − 5 = 3.
  3. Put the 10 back: 10 + 3 = 13.
More examples

17 − 4

  1. Leave the 10 in 17 alone.
  2. Take away from the ones: 7 − 4 = 3.
  3. Put the 10 back: 10 + 3 = 13.

15 − 12

  1. Take 10 from the ten: 10 − 10 = 0.
  2. Take the ones from the ones: 5 − 2 = 3.
  3. So 15 − 12 = 3.

Try it yourself

  1. 18 − 5 =

  2. 15 − 2 =

Watch out

  • You take away the ten as well when only ones are taken away: for 18 − 5 you say 3.

    The ten stays when you only take away ones. The answer is still more than 10.

Tips

  • The ten stays: only take away from the ones.

Exceptions

  • If you need to take away more ones than there are, like 12 − 5, you cross ten. Then use Bridge through ten.

Why it works

18 is one ten and 8 ones. If you take away 5, they come from the ones: the ten stays. If a 10 goes as well, take it from the ten and then work with the ones.