Money Percentages - Coupons?

I have a very simple question about how percentages are dealt with in money. Say for example I want to buy a book from ';Yahoo! Bookstore';, and I have a membership with them that gives me 50% off of everything I buy. For simple figures, lets assume this book is $100.00. Also assume that I have a coupon for 10% off any book from ';Yahoo! Bookstore';. You could do this in one of two ways, either...





a) subtract 50% ($50.00) from the book, then subtract 10% ($5.00) from the fifty dollars, ending up with $45.00.





b) combine the two, and have 60% off the book, therefore saving you $60.00, and making the book only $40.00.





Obviously, selection B saves you five dollars. My question is, is there some universal rule for this? Which method is considered correct?Money Percentages - Coupons?
Everybody does it chainwise; you'll get 55% off.Money Percentages - Coupons?
in most cases you would only get one or the other. ';offers can;t be combined'; '; no other discounts can be taken'; etc.