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**Unformatted text preview: **are chosen at a time (r n). The number of of n items chosen r at a time is written n P r. (n-r)! n! r n n n n = + = ) 1 )...( 2 )( 1 )( ( P r n (example) Basic Probability Permutations and Combinations:- Permutations (classes): To calculate the number of considering classes of similar items. A total of n items to be placed. n 1 items are the same of one class, n 2 are the same of the second class and n 3 are the same as a third class. n 1 +n 2 +n 3 =n The number of permutations of n items taken n at a time: ! n n n n! 3 2 1 n n ! ! P = (example)...

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