You just need to move on the vertical line corresponding to x = 30 students.
Start at the x-axis, for x=30 students, and move vertically upward until you reach the point, then move horizontally to the left, until the y-axis and do your best estimate of the y-coordinate reading.
The value is a little below 180 and quite above 170, then a good approximate is 180. It could also be 179, or 178. Almost sure a number greater than 175.
