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how would you judge which class performed best?

Leaf                                       Stem                                            Leaf
2                                               1
9                                               2
8                                               3
9                                               4
8   7                                          5                                              0   0   1   2   2
7   7                                          6                                              3   4   6
9   8                                          7                                              0   1   2   4   5
8   6   5   3   3                            8                                              9
9   2   1   1   0                            9                                              1   1   4   8
How would you judge which class performed best? What's the answer?? T.T Does it has to do anything with the mean and number of students?
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2 Answers

Just to add a little detail, the stem and leaf plot gives you the scores for each individual in each class. But the data are arranged in an interesting way that may be a little confusing if you are not used to looking at these plots. 
The stem is the tens position and the leaf is the ones digit for each individual score. If you see a number repeated on the same row then it means that more than one case exists with that score in that group. 
For a simple example let's take the first case on the left group. You will see a value of 2 for the leaf and a value of 1 for the stem. That means that one individual in that group had a score of 12. 
Let's try the next one down just to try to hit the point home. For the next value down on the group on the left the leaf is 9 and the stem is 2. That means one case had a score of 29 for the group on the left. 
Okay, so now if we move to the group on the right, you will see that in the first row there are two 0's and two 2's that start with the stem 5. That means two cases had scores of 50 and two cases had scores of 52 for the group on the right. That is how you would interpret repeated numbers on the same row (i.e., attached to the same stem). 
Now to evaluate which group performed better, you will have to rearrange the scores into complete numbers, add them together and find the mean for each group. If the question requires a test of significance, then you will need to take additional steps after calculating the mean for each group. If that is the case, it would seem that a t-test might be appropriate given the nature of your data. 
Best of luck. 
Good morning, Vivi!
Yes, this does have to do with mean.  In order to determine which class had the best overall average, you will need to add up every score for each class, then divide those totals by the number of values entered for each class respectively.
Hope this helps!