Let be the fraction of your customers that took the multi-buy. Then the total sales were and the total they paid was .But not everyone takes up the offer (some ppl prefer to just buy the one unit instead of two). In my data the average sell price for the week of the multi-buy promotion was $3.86
So how do i figure out the % of sales that were made were shoppers took up the multi-buy?
So the average price was the total received divided by the total sales:
Now the s cancel and we are told that the average price paid was so:
which you now solve for the (decimal) fraction of customers who took up the multi-buy, and multiply that by to get the percentage.