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It means that out of every four questions on the quiz, you answered one wrong. Or that out of every four bits of credit the teacher planned to award for accurate work on the quiz, your work was only...
80% of 75% is 60%. This is because 80% of 75 is 60.
75% of 13 = 9.759.75 as a percent = 975%
75 percent of 40 is 30, and 30 is the same as 0.3 percent