



 
Hi, If order is not important, for example 135 is the same as 351, then you want the number of ways of choosing 3 things from 6 things. This is called "6 choose 3" and it is written \[\left( \begin{array}{c}6\\3 \end{array} \right) = \frac{6!}{3! (63)!} = \frac{6 \times 5 \times 4}{3 \times 2 \times 1} = 20.\] If order is important then these are called permutations. You have 6 choices for the first digit, 5 choices for the second digit and 4 choices for the third digit so you have \[6 \times 5 \times 4 = 120 \mbox{ permutations.}\] Penny  


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