Answer:
the boy will appear to be moving at 10 m/s relative to the cyclist.
Explanation:
We need to use the concept of relative velocity (as vector quantities) due to motion in opposite directions.
If the cyclist moves at 9 m/s in one direction (let's assume positive direction in a linear axis) and the boy at 1 m/s in opposite direction, then we assign a positive sign to the 9 m/s and a negative sign to the velocity of the boy.
The velocity difference will then be:
| 9 m/s - (-1 m/s) | = 10 m/s
Then, the boy will appear to be moving at 10 m/s relative to the cyclist.