You can't exactly do that, but you can have Assignment Groups ignore the x number of lowest scores. So with your example if there are 15 assignments worth 5 points each, put them all in a single Assignment Group and set it to ignore the lowest five. Keep in mind that a blank grade is not a lowest grade, so all blanks would need to have 0s added at some point to ensure they get dropped.
This isn't exactly what you are looking for because I assume in your example a student could get 4 out of 5 on all 15 assignments and score 70/75, which would translate to 50 points. In my example dropping the lowest five, the student would only have 40 points.
To really accomplish you example some manual grading might be needed. Put all of those Assignments into an Assignment Group weighted at 0%, so they don't really count at all. They are just records. Then at the end of the semester create a single Assignment to hold their grade and enter their total points (max 50) into it.
Rick
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