We can safely assume we are dealing with an arithmetic scale because the type of scale isn't mentioned anywhere. The more right a point is on the scale, the higher its value is. We are given two points of the scale: 3 and 4. Because the scale is arithmetic, we know that 3.8 must lie on 4/5s of the length between 3 and 4 to the right of 3, which is exactly where point D is.
