This problem is about the distance between points, in this case, we have to unidimensional coordinates, that is, point O is at -2, and Point V is at 1/2.
It's important to know that the proper way to find the distance here is by subtraction, we need to subtract the final points and the initial point. In math, such subtraction is called "differential" or "variation", just a fancy name.
Therefore, the distance from O to B is 2.5 units.