Which is a better deal on a $345 item; 40% off or 30% off and additional 15% at the register
+36916 x = original price of item
option a 40% off means you pay .6 x .6 x 345 = $207
option b 30% off with 15% off that price at register
means you pay .7 * .85 x = .595 x = .595 (345) = $205.27
Option b is slightly better.
