How many different two-digit numbers can be formed using the digits 1, 3, 4, and 5. Repetition of digits is allowed.

8 ways

12 ways

16 ways

20 ways

13,14,15,31,34,35,41,43,45,51,53,54 so 12

We have 1, 3, 4 and 5

All are distinct numbers.

To form a two digit number, we have two places to fill with digits: _ _

The first place can be filled in 4 ways.

The second place can also be filled in 4 ways as repetation is allowed.

So total number of ways = 4*4 = 16

You can form 16 two digit numbers.