First, they determine the number of achievements that a has unlocked by dividing the total number of achievements by the percentage of achievements that she has unlocked:
48 achievements / 75% = 64 achievements
Then, they use this number to find the total number of possible achievements in the game by dividing the number of achievements that a has unlocked by the percentage of achievements she has unlocked:
64 achievements / 75% = 85 achievements
Therefore, the total number of possible achievements in the game is 85.
The work they used to get to this answer is shown below:
48 achievements / 75% = 64 achievements
64 achievements / 75% = 85 achievements