Algebra 1: determine if 4, -1, -6, -11, -16, … is an arithmetic sequence. if it is, give the common difference.

Yes, here is how we know.
First subtract the first two terms.
4 - ( - 1 ) = 5
Then compare it when you subtract the next pair
-1 - (-6) = 5 
Then the next
-6 - (-11) = 5
And lastly
-11 - (-16) = 5

Yes, it is an arithmetic sequence. The common difference is -5. Each term has been discovered by subtracting 5 from the previous.

When rates are expressed as a quantity of 1, such as 2 feet per second or 5 miles per hour, they are called unit rates. if you have a multiple-unit rate such as 120 students for every 3 buses, and want to find the single-unit rate, write a ratio equal to the multiple-unit rate with 1 as the second term.