Bathroom tissue is expensive, and everyone needs to pinch their pennies when they pinch off a loaf. Cell phone calculators don't have enough precision to calculate the per-unit cost of toilet paper. They also lack parentheses to do some necessary grouping to calculate:

PRICE / ( SHEETS PER ROLL * ROLLS )

Solution is below the fold.

This article about negative numbers has a bit of history. Negative numbers weren't understood by Europeans until the late 1400s, but they were discovered and used in India 700 years prior (the 700s?). The Arabs did not adopt the Indian notation, and thus, it was not transmitted to the Europeans.

Europeans discovered the Americas around the same time they learned to use -1.