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Add and subtract decimals mentally (with up to 3 decimal digits)

This lesson focuses on mental math and decimal addition/subtraction. First, we consider the sum 0.04 + 0.05, which many students mistakenly think equals 0.9 or 0.09. I show with number line jumps how it goes — we "land" at 0.45.

You can also easily see it if you make both numbers have the same amount of decimals. Write 0.4 as 0.40, and the problem becomes 0.40 + 0.05, or 40 hundredths plus 5 hundredths, which clearly is 0.45.

We solve other addition and subtraction problems using the same tactic or "trick" - adding decimal zeros until all numbers have the same number of decimal digits. After that the problems become so much easier, and we can solve many with just mental math!

In the end, we also solve two problems by writing the decimals one under the other (add/subtract in columns), carefully aligning the decimal points and the different places.

This lesson is meant for 5th grade onward. I hope it is of help!





See also

Math Mammoth Decimals 2 — downloadable worktext

Multiplying decimals and whole numbers: scaling — video lesson

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