See how extra credit raises your grade.
Extra credit works in points, and the crucial question is whether it lands in the numerator only or in both. When bonus points are added to what you earned but not to what was possible, thirty points on an 800-point course lifts an 80 percent to 83.75. When the instructor also raises the denominator — some do, to keep the scale honest — the same thirty points barely move the grade at all. Both behaviours are modelled, because assuming the wrong one is how students overestimate what extra credit will rescue.
Numerator only
New grade = (points earned + extra credit) / points possible x 100
Both sides
New grade = (points earned + extra credit) / (points possible + extra credit) x 100
Numerator-only is far more common, because the point of extra credit is to reward additional effort. If the syllabus describes the work as a graded assignment rather than a bonus, assume it raises the denominator too.
Look at the share-of-course figure. Extra credit worth under about three percent of the total points can lift a borderline grade a fraction of a band; it cannot recover a failing grade.