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Guiding Question: How can the standard algorithm help us solve problems with money and measurement?
The golden rule: line up the decimal
The golden rule: line up the decimal
Unlike whole numbers, where you just right-align everything, decimals must be aligned by the decimal point. This ensures you are adding "tenths to tenths" and "ones to ones."
If one number has more decimal places than the other (e.g., 12.5 + 3.42), add a "placeholder zero" to the end of the shorter number so they look the same (12.50 + 3.42).
step-by-step procedure
step-by-step procedure
Estimate First: Round your numbers to the nearest whole number to get a "ballpark" answer.
Example: 15.82 + 4.10 is about 16 + 4 = 20.
Stack and Align: Write the numbers vertically, lining up the decimal points.
Fill the Gaps: Add placeholder zeros if necessary.
Calculate: Add or subtract as you would with whole numbers, moving from right to left.
Drop the Point: Bring the decimal point straight down into your answer.
Verify: Compare your answer to your estimate. If your estimate was 20 and your answer is 19.92, you are likely correct!
common pitfalls to watch for
common pitfalls to watch for
Regrouping (Carrying/Borrowing): When subtracting, if the top number in a column is smaller than the bottom number, you must "borrow" from the neighbor to the left, even if you have to jump over the decimal point!
The "Invisible" Decimal: Remember that every whole number has an invisible decimal at the end (25 is the same as 25.0).
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