"Steve W" ha scritto nel messaggio
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Can someone please suggest a formula for summing the contents in a row -
for
a range consisting of the 12 columns immediately to the left of a
specified
column in that row

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Ciao Steve,
I think you can use:
=SUM(INDIRECT("rc[-12]:rc[-1]",FALSE))

That you can put in a reference to a definited name (es: sum12)
so you can use in worksheet =sum12

Or you can change style to reference from A1 to R1C1 ed use:
=SUM(rc[-12]:rc[-1])

ivano