These mistakes can't happen using scientific notation.its a known common error made by retards who never learned how to do sanity checks. They drop the units for some reason thinking thousand,million,billion is a unit not just shorthand for a number. They learned something in gradeschool about units cancelling out or some shit. People make this error all the fucking time. e.g
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Literally all you have to do if reverse the equation. Just do 1 billion x 500 = 500 billion ... oops thats only 500 people you retard.
What the fuck is a "sanity" check? I'm guessing checking your answer using common sense?