Answer:
Unit rate = 20.8 ft/s
Explanation:
We are given rate for 644.8 ft in 31 s
We need to find unit rate. We will calculate how many feet move in 1 second. 
When rates are expressed as a quantity of single unit.
Such as 2 feet per second (It means in 1 seconds move 2 feet) 
5 miles per hour ( In 1 hour move 5 miles)
We will calculate in 1 second how much feet. 
In 31 seconds = 644.8 ft 
In 1 second = 644.8÷31
Unit rate = 20.8 ft/s