Problem:

The following formula was created to sum a variable number of values in row 3 of Book1, Sheet1, starting at column A:
=SUM(OFFSET(\'C:\\My Documents\\[Book1.xls]Sheet1\'!$A$3,0,0,1,A2))
However, the formula only works correctly when Book1 is open.
When Book1 is closed, the formula returns #VALUE!.
How can we modify the formula so that it works regardless of whether Workbook1 is open or not?

Solution:

Use the SUM, COLUMN, and INDIRECT functions as shown in the following Array formula:
{=SUM((COLUMN(INDIRECT(\"A:Z\"))
IF(ISNUMBER(\'C:\\My Documents\\[Book1.xls]Sheet1\'!$A$3:$Z$3),\'C:\\My Documents\\[Book1.xls]Sheet1\'!$A$3:$Z$3))}