Moodle allows you to drop the lowest grade(s) from a category of activities/items in the gradebook.
Go to the course homepage.
Click Add category, at the bottom of the page
Name your category (e.g. Discussions)
Click Save changes
Enter a Weight or Point Value for the Category
Press Enter after entering a value
Move activities into your category
Click teh Edit drop-down next to the category
Click Edit settings from the drop-down menu
Click Show more...
Enter the number of lowest scores you wish to drop in the Drop the Lowest textbox
Click the Save changes button
In the category, under the Discussion total, you should see the formula
Use the form below to submit a ticket, we will get back to you as soon as we can!