Weighted Avg - using values from multiple sheets
 

I maintain a spreadsheet for a telephone hotline. We track total
number of calls received (column F) as well as average time to answer
the call (column J). To analyze the month's average time to answer, I
use the following weighted average formula: SUMPRODUCT
(J5:J26,F5:F26)/SUM(F5:26).

I have 2 questions relating to this:

1) Is the above formula correct to determine the average speed to
answer? I've assumed the weights to be the total number of calls
received.

2) I have a different spreadsheet for each month (Jan, Feb, etc.).
How can I properly show the weighted average as a Year-To-Date number?

Thanks for your consideration,

Joel McBride
Reno, Nevada