The class has 71 students
Let us find the number of student in each score
Between 0 and 10 on the frequency bar, there are 5 intervals
10/5 = 2
Each interval = 2 students
At least 80 means start from 80 and take all scores greater than it
From the graph at least 80 has two bars
Let us find their heights
The height of the bar of 80 is over 20 by 1 interval
So it has height = 20 + 2 = 22 students
The height of the next bar to 80 is over 10 by 1.5 interval
2 * 1.5 intervals = 3 students
So its height = 10 + 3 = 13 students
The number of students that scored at least 80 = 22 + 13 = 35 students