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It is not a matter of aligning the decimal points, but aligning the place value columns so that the ones are under each other, the tens are under each other, the tenths are under each other, etc which is the proper way to subtract. As the decimal points are between the ones and tenths columns, with those place value columns aligned, the decimal points are aligned.
Aligning the decimal points is an easy way to remember to align the place value columns, as with the decimal points aligned all the place value columns are automatically aligned.
What else can I help you with?
Why do you line up the decimal points when adding and subtracting?
you have to line up the decimal points because you need to keep each place value in the same column so when you add or subtract you wont make any errors
When subtracting or adding numbers in scientific notation why do the exponents need to be the same?
This is effectively the same as lining up the decimal points when adding or subtracting ordinary decimal fractions.
Do decimal points need to be lined up when adding and subtracting decimals?
Yes, unless you are an expert.
Do you line up decimal points when multiplyig?
Unlike addition, there is no need to line up the decimal points. Just multiply ignoring the decimal points. When you are done, move the decimal point of the result to the left equal to the number of digits after the decimal point for both multiplicands.
What do you need to line up decimal points before comparing and ordering numbers with decimals?
You do not, but it is easier.
