Discrete Mathematics

---------------------------------------------------------------COUNTING

219

as other digits can be any

of

the

10

digits. Thus we get a special case

of

the previous question with k

1

= 9 and

k

2

= k

3

= ... k, =

lO.

Thus, the answer is 9 · 1

0"-

1

•

7

.1.4

Permutations

During the party, we have already encountered the problem: how many ways can we seat n people on n chairs

(well, we have encountered it for

n = 6 and n = 7, but the question is natural enough for any

n).lf

we imagine

that the seats are numbered, then finding a seating for these people is the same as assigning them to the

numbers

1,

2, ... , n (or

0,

1,

... ,

n-

1,

if

we

want to ...