New answers tagged back-to-the-future
In Back to the Future Part II, how could old Biff have made it back to 2015? Answers most of your question, but the other part is simply that changes to the time stream in BTTF are not instant. The characters have enough time to affect the changes before they are erased from existence. Paradox averted.
It's simple. If they would dig up the first time machine it had been siting in the mine for 6 months, when doc got there in 1885. when Marty got there in 1885, the replacement parts to fix it for 1955 would not be available until 1985
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