Explanation: For Susan’s grade to be exactly 80 percent, she must answer .8(60) = 48 questions correctly. If she answered 90 percent of the first 50 questions correctly, then she answered .9(50) = 45 questions correctly so far, which means she must answer 3 of the remaining 10 questions correct for a grade of 80 percent. 3/10 = .3, which equals 30 percent.
