Probability

Question

Think about the tetris game,which uses pieces with seven different shapes, labelled as I,J,L,O,S,T and  Z. Assume that regardless of the prior sequence of pieces, all seven shapes are equally likely to be formed at any point in the game. Take into account the three-piece sequences like JLL,ZOS.ZSZ,III and so on that are seen while playing tetris. What is the probability that a T would appear in the second three-piece sequence but not in the first one?