Answer
The spring stretches by 20 cm
Work Step by Step
When Bob pulls on the end of the spring with a force of 200 N, then according to Newton's third law, the spring pulls on Bob with a force of 200 N. Also the spring pulls on the wall with a force of 200 N. According to Newton's third law, the wall pulls on the spring with a force of 200 N.
When the wall is replaced by Carlos, the situation is exactly the same from the spring's point of view. Therefore the spring stretches by 20 cm, just as it did in the first case.
