Students in a fifth-grade class were given an exam. During the next 2 years, the same students were retested several times. The average score was given by the model

f(t)=81-12log(t+1),0<t<24

where tis the time in months round answer to 1 decimal point.

(a)What is the average score on the original exam?_____________

(b)What was the average score after 6 months?______________

(c)What was the average score after18 months?_______________