If my memory serves me correctly, "Marty uses the time machine in Back to the Future to return a mere 15 minutes before Doc's assassination by terrorists at the beginning of the movie.", this is untrue.
He gives Doc a letter explaining what happens in the future just before he leaves 1955. The doc tells him he cant give information about the future and rips up the letter... But what happened, Doc actually did read the letter and avoids his death.
I do not recall Marty ever going back a mere 15 minutes before docs assassination.
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