trig
posted by jerson .
in an algebra class of 21 students the mean grade on a test was 87; in another class with 32 students the mean grade was 81. What is the combined mean of the two classed?
my teacher said it's 21 * 87 + 32 * 81/53= 83.4
but i'm not sure where the 53 came from..
thanks in advance

your adding the total number students from each class. 21 + 32 = 53