What motivates people?

I think rewards motivate people. For this experiment, rewards strongly motivate group A to complete the problem. However, I guess group B figure out the problem quicker than group A.

First reason is that people in group A think about the other thing. People in group Awould think that "I want to be the quickest!! I want to get rewards!!". They also think about the other people, like "how are their solutions going?", while they solve the problem. Thinking about the other people interrupt solving the problem.

Second reason is that people feel pressure. I think  if people try to do something quick and correct, like exams, they would feel nervous. It make people take more time to do it.

For these reasons, I guess group B completes the problem quicker.