If the slope is 3, it's actually 3/1 because of rise/run. So you find any point on the graph that's directly between two lines, like (-1,-5). Since the line is positive, you would go right one and up three. Then you see if that point is directly between two lines. The next point is (0, -2). If you did the same on the second graph, it wouldn't match up.
200 * 2/15 = the number of students that didn't read any books. This is 26.6, so 40 is not a good estimate 40 students is 20 percent versus 26 being approximately 13 percent.