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)=86−14log⁡(t+1),0≤t≤24\displaystyle {f{{\left({t}\right)}}}={86}-{14}{\log{{\left({t}+{1}\right)}}},\qquad{0}\leq{t}\leq{24}
where t\displaystyle {t} is the time in months. Round answers to at least 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 after 18 months?
 

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