2.1 Using number bonds to 10, 20, 100
Learning number bonds is an important skill which supports learners to be able to answer more difficult questions. For example, learning how to quickly calculate number bonds to 100 can help with bridging over 100.
A quick way of calculating 100 – 38 is to make the tens number up to 90 and the units number up to 10.
You can also say that 62 is the complement to 38.
Note that we have not used an ‘equals’ sign because these are not strictly mathematical sentences but they mirror what the learner may be saying in their head or aloud.
Similar quick ways can be used to find numbers that sum to 1000. For example, to work out 1000 – 159:
This method also works for reckoning the change from £10.00.