Go shopping

Hi today at school we talked about the "% " the teacher told us that there are a lot of "% " when you go shopping  because there are a lot of bargains.

He told us that if we want to buy a phone  it cost 100€,   but you know that the next week it will cost  a 20%  less, but  that week you can't buy it for any reason and you have to buy it in two weeks,  the bargain has finished and it costs 20% more.
How much is it going to cost that phone each time?
The first time i 100€, the second time is 100-20%(80€) and the third time is the 80+20%(96€).
There is another option which is that the first price is takes as their reference . In that case the last price would be 100+20%(120€)

Marcel Rovira