why in adding and subtracting scientific notation, you need the same power of 10 whereas in multiplying and dividing scientific notation, you can have two different powers of 10?

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it will cost dollar $540

the living room is 3 which means if we divide 325/3 then we get $108

therefore if 1part of the living room cost $108 and 3 costed $325

then total 5part of the room will cost 5*108 which gives $540