If I understand what the question might be, they're asking for the length of BC.
4 points (A, B, C, and D) make up the number line, with a total distance of 3.75 inches.
Line segment BC makes up one-third of the line, so the length of BC is 3.75 divided by 3. When you solve, you'll get 1.25.
So the length of line segment BC is 1.25 inches.