When you read these word problem questions, look for some key words. In this word problem, it asks which juice costs less PER OUNCE. When you see that, then you know you will take the dollar amount and divide it by the ounces.
Cranberry Juice
$5.76 / 64 ounces = $0.09 per ounce
Apple Juice
$1.80 / 15 ounces = $0.12 per ounce
So the cranberry juice costs less per ounce. Hope this helps! :)