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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
Because when adding or subtracting, the operations must take account of the place values of individual digits in the numbers. When multiplying, only the overall order of magnitudes are relevant for placing the decimal point in the product.
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.
Yes.
The simplest way is to line up the numbers so that their decimal points are aligned, and corresponding digits are also aligned according to their place value.