For rates involving prices, we're usually interested in the price per item, so we divide the cost by the number of items to form the appropriate rate. Simplified, it is the unit rate.
ratio = $140/(7 calculators) = $20/calculator = unit rate
___
A "unit rate" is a rate with 1 (unit) in the denominator.
"per" can usually be considered to mean "divided by" in this context. As in "price per item" means "price divided by items".
