Batteries which normally sell for $1.89 a pack are on sale for $3.40 for to packs approximately what percentage markdown does this represent perfect per pack

3.40 for 2 packs = 3.40/2 = 1.70

percent decrease = (original number - new number) / original number ...*100
                            = (1.89 - 1.70) / 1.89* 100
                            = 0.19 / 1.89...* 100
                            = 0.1 * 100
                            = 10 % <=== there is a 10% decrease per pack

Delta airlines or untied