1. We are creating 2 digit numbers using the numbers 1-5.
We can choose the first number from 5 choices. So options for the first number = 5C1 or 5. Then we have just as many choices for the second number so 5C1 or 5 again.
So two digit numbers made up of 5 choices = 5C1*5C1=25. Then we have to go back and subtract out the duplicates, and we can intuitively see there are 5 of those (11,22,33,44,55) so 25-5=20
2. This is the same problem except that we don't have to subtract the number of repeating numbers (11, 22, etc) so 5C1*5C1=25