A sociologist determines that a foreign-language student has learned N(t)=20t-t^2 vocabulary terms after t hours of uninterrupted study.
What is the maxmimum rate in terms per hour at which the student is learning?
Ok.. this is what I did, but the back of the book has a different answer than my final answer.
My answer: 100
Back of book: 20
So, what did I do wrong?