If you weight various items or categories by  particular percentages of the overall course grade, use the Weighted mean of grades aggregation type. After selecting this type, a weight column will appear where users may assign percentages to items or categories, i.e. Exam 1 is 25%.

Step-by-step guide

1. From the main course page, select Gradebook setup from the gear icon.
   
   
   
   
2. Click the Edit > Edit settings link at the top of the page.
   
   
   
   
3. Toggle or confirm that Aggregation is set to Weighted mean of grades.
   

1. Click Save changes at the bottom of the screen and the Weight fields will display. Enter the weights for the categories or grade items, and click Save changes at the bottom of the screen.