Given a math problem with context and a question and 5 answer choices, the task is to provide the correct answer choice based on the problem. You must choose one of the given answer choices by letter: a, b, c, d, or e; anything else is invalid.
Q: Problem: if 15 students in a class average 75 % on an exam and 10 students average 90 % on the same exam, what is the average in percent for all 25 students ?
Options: a. 80 %, b. 81 %, c. 82 %, d. 83 %, e. 84 %
A:
b