When we added whole numbers, we right-justified the values and added down the columns. Lining up the last digit in each addend put all the digits with the same place value in the same column.
Now that we have extended the place value system to include decimal fractions, we can no longer line up the last digit in order to line up the place values, because the last digit of the decimals does not necessarily have the same place value the way it was with whole numbers.
Instead, we need to line up the decimal points to line up the corresponding place values, regardless of how many decimal places an addend has.
Example: Find the sum 4.5 + 34.98 + 6.
For the numerals to line up with the correct place value, we write the problem as
Notice that the 6 is six ones, and therefore can be written as 6. Also, we can fill in the end decimal places with zeros if we want to make all of the addends have a value in each place value position. This is not necessary, but sometimes it helps us to keep the place values lined up properly.
Just as with adding whole numbers, when a group of 10 is collected in one place value, that group of ten is recorded as 1 in the next place value to the left.
The sum 4.5 + 34.98 + 6 is 45.48.
Shawn's daily water useage over three days was 5.7 cubic feet, 12 cubic feet, and 9.35 cubic feet. How much water did Shawn use over the three days.
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Date last modified: June 26, 2010.
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By Kris Montis and Timothy Peil
