huge doubt on anomaly detection
@Ram , you have correctly said. You might not need any algorithm to detect these known abnormality. 

1) But even if u apply K-means, It will give you same result and will do it in automatically way.

2) Seems like you have just 1 variable -total ectivities, things get dirtier when you have multiple variables then you might have to look into LOF/ABOD other algorithm after identifying what exactly is abnormality for you.  Have a look here-

https://machinelearningstories.blogspot.com/2018/07/anomaly-detection-anomaly-detection-by.html

3) In ML , many a times you even do not know what exactly is an abnormlaity. Here you assume high activities is abnormality. depending on problem statement we need to have some unsupervised algo to identify possible abnormalities.