Sayan is teaching a small undergraduate class which has 24 students. He periodically calls on students to answer questions in class (for class participation) and he chooses these at random. He has already called on 6 of them (so 18 not yet called upon). Today he first selects a group of three students at random amongst all 24 of the students and asks them to work out a problem on the board (as a team). Then he selects another group of 3 students at random to work out the next set of problems. Find the probability that in this selection none of the students had been called before.

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