Description is "Write a SQL to get the cumulative sum of an employee's salary over a period of 3 months but exclude the most recent month.".

For id=1 and month=4, the list of salary is

(id=1,month=2,salary=30),

(id=1,month=3,salary=40) and

(id=1,month=4,salary=60)

so, the sum is 130 not 150