Hello, discovery!

Here's the second one . . .

How many numbers between 2000 and 8999 (both inclusive)

donothave four different digits ?

There are numbers from 2000 to 8999.

We'll find the number of numbers thatdohave four different digits.

. . The first digit has 7 choices:

. . The other three digits are any permutation of the remaining 9 digits: .

. . Hence, there are: . numberswithfour different digits.

Therefore, there are: . numbers that donothave four different digits.