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Decimal place value and adding decimals, up to 2 decimal digits

I explain the basics of decimal place value: how decimals with tenths and hundredths (1-2 decimal digits) are FRACTIONS. We look at several examples of how to convert between fractions and such decimals.

Then we look at at common student misconception about ADDING decimals. Many students think that 0.5 + 0.06 is 0.11. They feel the decimal parts work just like whole numbers — as if the decimal parts were numbers living on the other side of the decimal point.

I show using number line jumps how that is not the case. We get that 0.5 + 0.06 is actually 0.56. Another way to see that is to place a decimal zero after 0.5 so it is written as 0.50. Then the addition is 0.50 + 0.06, or 50 hundredths plus 6 hundredths, which is 56 hundredths.

This little "math trick" — adding decimal zeros to the end of a decimal - does not change the value of a decimal, yet allows us to easily add decimals of different "lengths" (different amounts of decimal digits) using mental math.

This lesson is great as a review for 5th grade, but works well also in 4th grade math.





See also

Add & subtract decimals using mental math — video lesson

Math Mammoth Decimals 2 — downloadable worktext

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