# rate of change for exam information

we are given following problem: Students are given an exam and retake the exam later. The average score on the exam is

 S = 80 - 14ln(t + 1)


where t is the number of months after the exam that the student retook the exam. At what rate is the average student forgetting the information after 6 months? i think that we should take derivative of S with respect t and put 6 into derivative .in this case dS/dt=-14/(t+1) if we put t=6 we get -(14/7)=-2 so it means that about 2 student forget information after 6 month?am i right ?maybe i am confused of some condition of this problem if so please help.

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aaa yes it is clear i have mixed in each other thanks @anon – dato datuashvili Aug 8 '11 at 15:20
@Andre: Oh yes, please excuse my brain for that. – anon Aug 8 '11 at 15:45

The calculation is correct. The conclusion about "$2$ students" is not.
The rate of change of the mark at time $t=6$ (months) is indeed $-2$.
In informal language, we can conclude that the mark at $t=6$ is decreasing at the rate of $2$ marks a month. (Increasing at rate $-2$ would sound funny to most people, but is certainly correct.)