We find the average by adding up all the students and dividing them by the number of classes she has. 19 + 13 + 19 + 9 + 14 + 15 is 89, and that divided by 6 is approximately 14.8, rounded to the nearest tenth.
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A 90º degree angle is a right angle. Anything below is an acute angle and anything more than 90º is an obtuse angle.