Write an equation for the nth term of the arithmetic sequence. then find A10. 13, 9, 5, 1

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.

What am i doing here. add?