Calculating the Average Transaction Value

Hello,

I am trying to calculate the average transaction value on a report I'm building.

My data is displayed by product sold, i.e. there are multiple lines for each transaction, depending on how many items were sold on that transaction. Each line has the transaction number and the amount paid for that product.

Basically, I want to do:    Sum of items sold  /  Count of unique transaction number

Please help!! :) 

Thanks,