Exclude Weekends from Formula

I am trying to calculate total hours that an item is "open" without including weekend hours in the formula. I have been using the following formula field to calculate the total hours from start to end, but I'm not sure how to exclude weekends from being included in the hour total.

Here is the formula that I am currently using:

((Date_Approved - Added_Time) / (100000 * 60))

Any help would be much appreciated!

Thanks