Answer:
Exponential decay by 9% each cycle (n).
Explanation:
The function y = 51(0.91)^n is an exponential decay function. The term (0.91) is less than 1, so when multiplied by anything, the result will be less than the original number, by 9% [(100-91)/100]. The power n, represents the number of cycles (e.g., days, minutes, years, seconds, etc.). Each succeeding cycle reduces the previous number by the same amount of (0.91).
See the attached graph.