Sum across variable columns

Dear All,

I have a little problem I'm trying to overcome, and I'm trying not to use named ranges.

In the attached file, I have a simplified version of what I'm trying to do. I want to be able to input variable numbers of weeks into Cell B7 and have the Summed results appear against each Item below.

As an example, the result I am looking for if I entered '3' in B7 would be the sum of 3 weeks (columns B to D). If I entered '4' then 4 weeks (columns B to E).

Does anyone know of a formula I could use to achieve this?

Many Thanks,
Nick