Subtract the percent of your grade the exam is worth from 100 to find what percent of your grade is already determined and divide by 100 to express the result in decimal form. If your exam is worth 20 percent, for example, (100 - 20)/100 = 0.8.
Multiply the result by your semester grade expressed as a percentage to determine what percent of the total possible grade you have already achieved. If your semester grade so far is 92 percent, for example, 92*0.8 = 73.6 percent.
Subtract the result from the minimum total percentage you need to achieve the grade you want. If you need 88 percent to get an A, and you have achieved 65 percent without the exam, 88 - 73.6 = 14.4.
Divide the result by the percent of your grade the exam accounts for and multiply by 100 to find the percentage score you need on the exam. If the exam is 20 percent of your grade, (14.4/20)*100 = 72 percent. In this case, you need to score at least 72 percent on the exam to earn an A.