What is the average number of books read by a group of 20 students, given that each student read 5, 8, 6, 10, 7, 9, 5, 4, 8, 7, 6, 9, 7, 6, 5, 7, 6, 8, 9, and 4 books, respectively?
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Math question: Evaluate log(base 3) (81) + 2 log(base 4) (16) - log(base 5) (25). Simplify the expression using logarithmic properties.