Male and female undergraduates (N=144) in the departments of science and technology and departments of liberal arts observed two cars traveling in the same direction for various duration on one of four CRT displays in a class. Then, they chose the car that they believed had run longer and rated confidence of their choosing. There were two sessions each of which consisted of nine problems. Before each session, they were asked what they were going to pay attention to and how they were going to solve the problems. Between the two sessions, they discussed about how to solve the problems in pairs, for five minutes. Main results were as follows: (a) They were more likely to use the knowledge ""duration=temporal end point-temporal start point"" than the knowledge ""duration=distance/speed."" (b) Men used more often both kinds of knowledge with planning than women did. (c) There were no effects of the discussion.