25. Students participating in a psychology experiment attended several lectures and were
given an exam. Every month for the next year, the students were retested to see how
much of the material they remembered.
The average scores for the group are given by the human memory model,
f(t)= 90-15 log10 (t+1), 0≤ t ≤ 12 where t is the time in months. When did the
average score decrease to 75?