Final answer:
To predict Brad's final exam score, calculate the average of his five test scores, determine the percent of his average represented by two-thirds, and set up an equation to solve for the final exam score needed for a B grade.
Step-by-step explanation:
To predict the final exam score for a student who scored a 90 on the third exam, we can use the given information. The average of Brad's five test scores is the two-thirds of his final grade, and the remaining one-third is the final exam score. We can start by calculating the average of his five test scores by adding them up and then dividing by 5: (87 + 71 + 72 + 74 + 86) / 5 = 78.
Next, we can calculate the total number of points available for his final grade. Since the average counts for two-thirds of the final grade, it means that the remaining one-third represents [(100/2) - 2/3] = 50% of the final grade. If Brad needs a final grade greater than or equal to 80, we can set up the equation: (78 * (2/3)) + (x * (1/3)) >= 80, where x represents the final exam score. To solve for x, we can rearrange the equation: (78 * (2/3)) + (x * (1/3)) - 80 >= 0. Solving this equation, we get x >= 82. Therefore, Brad needs to score 82 or higher on his final exam to get a B.