A new pencil is 19 centimeters long. Levi uses it for a month. The pencil now measures 8centimeters.

Answer

11/d cm per day

Q:What is the total number of days Levi used the pencil? A:Let's assume Levi used the pencil for d days.
Q:What is the rate of decrease of the pencil length in cm per day? A:The rate of decrease of the pencil length in cm per day is (19-8)/d = 11/d.