If you do extra credit to make up for a bad grade, your resulting grade is determined as follows: Your grade on the make-up, expressed as a percentage, adds that percentage of the difference of your original grade and one hundred to your original grade. In other words, if your original grade is O, your grade on the extra credit is E, and your resulting grade is R, then R = O + (E/100)(100-O). For example, if you got a 40 on your first try, and then got a 50 on the extra credit, your resulting grade would be 40 +(50/100)(100-40) = 40 + 30 = 70.
This means that you have nothing to lose by doing the extra credit.