base - calculate a YTD average

slightly more complex... but what i want is some kind of base figure sum. i'll explain...

i've got data for each sales week, 1 thru to 52.

I want to calculate a YTD average.

so week 1/week1

thru to week 1/week 32...
week 1/week 52 etc

any formula i can do where it treats week 1 as the base figure, rather than do it all manually. if that makes sense?