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

$3.40 for two packs is $1.70 per pack  ($3.40/2)

The amount of change $1.89 to $1.70 is $ .19

.19       which we put over the original amount
1.89

and get approximately .100529..      So I would say about 10%

Ujjg