One eighth of the class graduated with an A average, 1/2 got a B, and the rest of the class had a C average. In a class of 800, how many had a c average

1/8  of the class graduated with an A average
1/2= 4/8 got a B
the rest of the class had a C average = 3/8

In a class of 800

800/8 = 100

so 

1/8  of the class graduated with an A average = 100
1/2 got a B = 400
the rest of the class had a C average = 300

answer
300 had a C average.