the question is:

"Sum of a series of numbers entered by the user until the value 999 is entered but 999 should not be a part of the sum"

at first i thought the question was asking for the sum of user entered numbers until the sum reaches 999. after reading it more carefully is it right to assume that if the user enters the number 999 that is when the program should stop calculating the sum and print it?

You're second thought sounds correct. Loop inputting a number each time and add it to the sum until the current number entered is 999, then stop input and output the sum making sure to either not add 999 to the sum or subract it off after you break the loop.