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

\[\left( \begin{array}{c}6\\3 \end{array} \right) = \frac{6!}{3! (6-3)!} = \frac{6 \times 5 \times 4}{3 \times 2 \times 1} = 20.\]

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