Mr. Yoshi graded 60 out of 75 papers. Percentages are the ratio of a portion to the whole, so Mr. Yoshi graded: 60/75 = 4/5 or 80% of the papers. Now, we could subtract: 100% - 80% = 20%, but let's calculate: the student graded 75-60 = 15 of the papers. 15/75 = 1/5 = 20% of the papers.