Summary
By Karen Go Soco
Using the bridging strategy to subtract a single digit
When subtracting a single digit, it is not uncommon for learners to rely on their fingers to count back in ones. This is particularly true for learners who have dyscalculia or other maths difficulties and mistakes can often occur due to a lack of confidence and accuracy when counting back across the tens boundaries (e.g. 42, 41, 40, 39).
Bridging is a strategy that can be used to subtract as well as add (See previous video on Bridging to add here- https://dyscalculianetwork.com/fixit-with-karen-bridging-10-to-add-for-dyscalculia/ )
Just as when bridging to add, there are prerequisite skills needed before using the bridging back through 10 strategy to subtract. These skills are:
- Fluent recall number bonds for 10
- The ability to decompose numbers up to 10. For example, knowing that 8 is composed of 4 and 4, or 5 and 3, or 6 and 2, or 7 and 1.
- Know how many must be subtracted to reach the previous multiple of ten. For example, if we start with 14, we must subtract 4 to reach 10.
Using the bridging back through 10 strategy involves decomposing a number into 2 parts to make subtraction easier. Firstly, we subtract enough to reach 10, then we can apply knowledge of bonds for 10 to subtract the remaining amount. In this way, we don’t need to rely on counting which takes up a lot more working memory than using known facts.
The bridging strategy can also be useful when using the written column method, as this frequently involves subtracting from ‘teen’ numbers. Once secure, it can also be used to mentally subtract a single digit from any number with two or more digits. When numbers are close to a hundreds or thousands boundary, bridging can also be used to subtract across the boundary. Bridging can even be applied when working with decimals, allowing us to bridge back across a whole number.
See how to bridge back across ten, and also across multiples of ten, in this video.
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