Here is the solution if the unique weight is number 7 and it is Lighter
than all others, followed by solution if it was Heavier than all of the others.

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1st WEIGH

Solver would place 1 2 3 & 4 on left side of scale and 5 6 7 8 on right side.
He now knows unique weight is either 1 2 3 or 4 being HEAVIER or
5 6 7 or 8 being LIGHTER

2nd weigh

Solver now places 1 2 & 6 on LEFT side of scale and 3 4 & 5 on RIGHT side.
Solver would see that the scale is in balance. So Unique weight must be either 7 or 8 as Light.

(see 1st Weigh results)

3rd WEIGH

Solver now places 7 on LEFT side and 8 on RIGHT side.

Solver would see that left side is Light so unique weight is 7 LIGHT.

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Assume instead that 7 is Heavy.

1st WEIGH

Place 1 2 3 4 on left and place 5 6 7 8 on right

Either one of the above is Light or one is Heavy.

2nd WEIGH

Place 1 2 6 on Left Place 3 4 5 on Right

Solver would see that scale is in balance so must be either 7 or 8 is Heavy.

3rd Weigh

Place 7 on Left. Place 8 on Right

Solver would see that Left is Heavy so Unique weight is 7 as Heavy

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