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# Bridging decades

Decades are multiples of 10 (i.e. 10, 20, 30...). Bridging decades involves using multiples of 10 as landing points for adding and subtracting.

For example, 28 + 7 = ? might be solved by bridging 30, with 28 + 2 = 30, then 30 + 5 = 35.

Similarly 35 – 7 = ? might also be solved by bridging 30, with 35 – 5 = 30, then 30 – 2 = 28.

Success in bridging decades is built on a foundation of knowledge about place value structures (e.g. 30 + 5 = 35) as well as number facts for 10 (e.g. 8 + 2 = 10) and numbers less than 10 (e.g. 5 + 2 = 7).

You can read more in the article *Mental Methods Moving Along* on the AAMT website.

## Up through ten

Up-through-ten strategies use decades as landmarks to solve harder calculations. For example, 9 + 7 = ? can be solved by calculating 9 + 1 = 10 then 10 + 6 = 16.

## Back through ten

Back-through-ten strategies involve using decades as landmarks to solve harder calculations. For example, 52 – 8 = ? can be solved by calculating 52 – 2 = 50 then 50 – 6 = 44.