wrote...
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You need to use a 3 dimensional reference, eg: =SUM('A:C'!A5), note
the ' before and after A and C, this denotes the 3D reference. . . .

No, the colon (:) between tokens to the left of the exclamation point (!)
denotes the 3D reference. The single quotes are a good idea all the time,
but they're only needed when worksheet names contain spaces.