At 3:29, it is said that there are m+k-1 spaces, how exactly it came from

#1

At 3:29, it is said that there are m+k-1 spaces, how exactly it came from

#2

Actually its not m+k-1 spaces.
When you have to put k items at m positions, you have formula C(m+k-1,k-1).

#3

I hope I’ve cleared your doubt. I ask you to please rate your experience here
Your feedback is very important. It helps us improve our platform and hence provide you
the learning experience you deserve.

On the off chance, you still have some questions or not find the answers satisfactory, you may reopen
the doubt.