The syntax for VLOOKUP is as follows:
VLOOKUP(lookup_value, table_array, col_index_num, [range_lookup])

Is there a way to have a dynamic "col_index_num" instead of a static
value?

For instance, in the formula:
=VLOOKUP($A15,'Sheet1'!$A:$AT,20,0)
is there a way to make the "20" a dynamic value such that if I add or
delete columns before the 20th one then the col_index_num will adjust
itself accordingly?

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tb