It counts lines and maintains a sum of the values seen to date. When the key changes, it prints the average, then resets the line count and the sum.

1    196       -> count = 1   sum = 196
1    190       -> count = 2   sum = 196+190
1    200       -> count = 3   sum = 196+190+200

 key changed, so print average, and reset count and sum

2    20        -> count = 1   sum = 20

 key changed, so print average, and reset count and sum

3    25        -> count = 1   sum = 25
3    19        -> count = 2   sum = 25+19
3    39        -> count = 3   sum = 25+19+39

 key changed, so print average, and reset count and sum

4    40        -> count = 1   sum = 40
4    41        -> count = 2   sum = 40+41
4    45        -> count = 3   sum = 40+41+45

 eof, so print average
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