It uses a pair of scales to get the answer (5,4)

I believe the answer is:

slide rule

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'it uses a pair of scales to get the answer' is the definition.
The definition suggests a singular noun which matches the answer.

This is the entire clue.

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(Other definitions for slide rule that I've seen before include "Old calculator" , "Logs on here" , "Obsolete calculator" , "old computer" , "Instrument for rapid calculations" .)

I've seen this clue in The Globe and Mail.

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