Sorry to continue my question, I clicked on the wrong button I think

An example of how I have set-up my spreadsheet is as follows:

Week Week No. Day
Date Task
15/10/2007 1 Monday
15/10/2007 Computer shopping
1 Tuesday 16/10/2007
1 Wednesday 17/10/2007
1 Thursday 18/10/2007
Clothes shopping
1 Friday 19/10/2007
1 Saturday 20/10/2007
1 Sunday 21/10/2007
22/10/2007 2 Monday 22/10/2007
2 Tuesday 23/10/2007
2 Wednesday 24/10/2007
2 Thursday
25/10/2007 Computer shopping
2 Friday 26/10/2007
2 Saturday 27/10/2007
2 Sunday 28/10/2007

I want excel to calculate how many weeks it took to complete 1 task.
For example computer shopping spanned over two weeks, whereas clothes
shopping only spanned over 1. Does anyone have any suggestions?

Thanks for your help!