OK this is doing my head in

can some one please help me out

basically I have two interests to work out, weekly and daily

The sales total is £81,588 for an 18 week period, based on 6 day selling weeks thats 108 days

The daily interest rate is 4.4%
The weekly interest rate is 4.5%

When I work out the two interests I get a difference of about 20p!!!!

If anyone can explain or show me the right way to do it that would be great as I am running out of time