1. **State the problem:** An athlete runs the same distance $x$ km on the first and second days. 2. On the third day, he runs 4 times the distance of the first day, which is $4x$ km. 3. On the fourth day, he runs 8 km. 4. The total distance run over the four days is 40 km. 5. Write an equation representing the total distance: $$x + x + 4x + 8 = 40$$ 6. Simplify the left side: $$6x + 8 = 40$$ 7. This matches option B: $6x + 8 = 40$. **Final answer:** Option B: $6x + 8 = 40$