What is your answer?

    The trail goes to the left or to the right.
    The trail doesn't go to the left.
    So the trail goes to the right.

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1 is correct!

    The trail goes to the left or to the right.
    The trail doesn't go to the left.
    So the trail goes to the right.

This is valid. It has this valid form: "A or B, not-A, so B."

I was hiking on the Appalachian Trail in West Virginia during the great snowstorm of 1993 (which killed over 150 people). The trail was covered by several feet of snow and was hard to follow. I used the above form of reasoning many times as I tried to get back to civilization. The premises represented "educated guesses" rather than firm knowledge. Reasoning can be useful even if the premises aren't known with certitude.

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2 is wrong. Please try again.

    The trail goes to the left or to the right.
    The trail doesn't go to the left.
    So the trail goes to the right.
    { 1 } - Valid
    { 2 } - Invalid

Where else could the trail go? The premises exclude any other possibility.

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