"Quantity of a Product" depending on a previous dropdown number

hello,
I need to set the "Quantity of a Product" depending on a previous dropdown number?
I wish to multiply the price of a product by the number of days that have been previously selected in a dropdown.

I need to multiply the "price of a product" by the number of days that have been previously selected in a dropdown.

the days are just from 1 to 7 (1 week) and you can previuosly select it from a dropdown (1-2-3-4-5-6-7).
the product is a single product.

for exemple:
I need to multiply the product "1 ski pass" by 3 days (and 3 days was previously selected on a dropdown inside the form)

how many days do you want to ski? (1-2-3-4-5-6-7) >> if you select 3 days the price of the product should be multiplicated by "3".

thanks!

my email is albert@dobleclic.biz