help with b. it is saying 121 is incorrect.

Nationwide Auto Parts uses a periodic roview inventory control system for one of its stock nems "The review interval is is weeks, ond itso leas time for recelving the materialt orderod from its whole a. What is the averapo and the standard devation of domopd during phe protoction intorval? The averape demand durng the protoction interval is 1500 units. (Enter your response as an integer)