To summary data

hi all,

my data is here,
--------------------------------------------------------------------------------------------------
data

  clss number
1    A      1
2    B      2
3    C      3
4    A      4
5    B      5
6    C      6
7    A      7
8    B      8
9    C      9
-----------------------------------------------------------------------------------------------------
I want to this format

clss number
A      12
B      15
C      18

is there a way to solve this problem?

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