Because if you don't line them up by the decimal, you could see a 9-3, when in fact it could be 9-30 or 90-3 accidentally. Mistakes happen. best to keep it simple
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Yes.
You subtract decimals together.
Yes, unless you are an expert.
line up the decimals together
You need to line up decimals for additional and subtraction. For multiplication, you don't need to line them up - in fact, it would serve no useful purpose.