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 $$f(t) = 83 - 13 \log_{10}(t+1), \qquad 0\leq t\leq 24$$ where $t$ is the time in months.

(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 after 18 months?