Weekly sales tracker - weekly average sales amount

Good afternoon, first post here. I normally google any issues I'm having but cant seem to find an answer to this one so hope someone can help me.

We write up our weekly sales on a whiteboard in the office but I have taken it upon myself to create a spreadsheet for this so we don't have to stand there with a calculator every week.
I have written a table showing each week No. with the amount sold each week i.e. Wk32 £48,456.98 - this value is taken from a separate tab which shows each sale individually.
At the bottom of this I have our total to date.
I Would like to show what our average weekly sales is going by our total to date divided by the number of weeks.
I cannot for the life of me come up with a formula to do this, all I can come up with is to manually change the week number each week.
i.e Weekly Average Sales =(B55/35).
Is there any way i can make a formula to divide cell No. B55 with the number of weeks data has been entered in without having to do it manually each week?
Sorry if this is hard to understand, I'm struggling to word this in a way someone that isn't looking at my spreadsheet can understand.

I have attached a dummy copy of this spreadsheet to clarify.

As I understood you need this formula:

=B55/COUNT(B3:B54)

if it works for you let me know. good luck

Originally Posted by barbareen

As I understood you need this formula:

=B55/COUNT(B3:B54)

if it works for you let me know. good luck

Worked Perfectly. Thank you very much!