Recalculate average after dropping lowest scores.
Dropping the lowest score raises the average by more than people expect, because it removes the outlier from the numerator and shrinks the denominator at the same time. Ten scores summing to 780 average 78; drop a 42 and the remaining nine average 82 — a four-point gain from one assignment. Working from the sum rather than a list of scores keeps the input short, and the drop count is capped so at least one score always remains to average.
After the drop
Average after = (sum of all scores - dropped scores) / (count - number dropped)
Gain
Improvement = average after - average before
Only when all the assignments are out of the same maximum. If they differ, the lowest percentage is the one to drop, not the lowest raw number — a 42 out of 50 beats a 61 out of 100.
It is a poor bet. The drop is insurance against your worst result, and spending it deliberately leaves nothing to cover an illness or a genuinely bad week later in the term.