Question 99369
Look at the total number of students. It is 306 and from this you can tell that to get rid
of the "6" in that total you will need 6 classes with 31 students. 6 times 31 is 186 students.
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So with 186 students in the 6 classes of 31, the remaining number of students is 306 minus 186
and that difference is 120 students. This group of 120 students is divided into 4 classes with
30 in each class.
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So the answer to your problem is that there are 4 classes with 30 students in each class and
6 classes with 31 students in each class. This results in 120 students in the classes 
having 30 students each and 186 students in the classes having 31 students in each for a 
total of 120 + 186 = 306 students.
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Hope this analysis helps you to see where the answer comes from.