Calculate prices by weeknumbers

I need some help with creating a formula that can calculate the price, depending on which week the purchase was made.

We’re selling weekly courses from February till July (23 weeks in total, €15,- each) The full program is €345,- When customers book a year in advance they get a €20 discount. I got up to the point where I made a formula that works and kinda goes like this:

IF weeknumber < 5, the price is €345
IF weeknumber > 28 the price is €325

Since people can join our courses at any giving time. I would like to add a formula that calculates the price depending on the week they joined. I’m looking for a formula that kinda goes like this:

IF weeknumber is between 6 and 27, the price is €420 - (weeknumber x €15)

Unfortunatly my coding isn’t that good yet… If anyone could help me out combining all of this in one formula, I’d feel blessed!

I believe your formula will be something like this:

IF({weeknumber}>28, 325, IF(AND({weeknumber}>6, {weeknumber}<27),420-({weeknumber}*15),345))