I need a formula which will add £15 out every 7th day of a calendar . Example Advance Purchase kicks in every 7 day and rate of the day reduces by £15.

Example : Column A in a full month ,day 1 to day 7 is same rate for selling a hotel room but on the day 8 the rate changes to less £15 in column B .
So Flexible rate is in Column A and Advance purchase rate from 7th Day is less than £15
what formula can i use to automate below

01-Oct £100 £100
02-Oct £100 £100
03-Oct £100 £100
04-Oct £100 £100
05-Oct £100 £100
06-Oct £100 £100
07-Oct £100 £100
08-Oct £100 £85
09-Oct £100 £85
10-Oct £100 £85
11-Oct £100 £85
12-Oct £100 £85
13-Oct £100 £85
14-Oct £100 £85
15-Oct £100 £85
16-Oct £100 £85
17-Oct £100 £85