A rate can express how long it takes to do something.
To drive 50 inches in one minute is to drive at the
rate
of 50 in./min.
This fraction (50/1) expressing the rate has units of distance
in the numerator and units of time in the denominator.
When the denominator of the "rate" fraction is a "single unit of one" (such as 1 minute), the rate is referred to as a unit rate. The most common type of rate is "per unit of time",
such as "miles per hour" or "inches per minute". 
When working with rate, the words "per, for and each" are often used to separate the units. 
Symbols such as / or @ may be used when discussing rates. 
Unit rate  a rate with a denominator of 1. 
