Use comma to split numbers 
All the three-digit numbers are written in the ascending order, from left to right, in a row such that each three-digit number is separated from the following number by a comma. How many such commas are present for which the digit immediately preceding the comma equals the digit immediately following it?


What I got:
The numbers should be in type $100, 101, ..., 999$. Such equal digits can only be $1, 2, ..., 9$.


*

*$1$: $10$ in total, i.e. $(101,102), (111, 112), ..., (191, 192)$

*...

*$8$: $10$ in total, i.e. $(808,809), (818,819), ..., (898,899)$

*$9$: $9$ in total, i.e. $(909,910), (919,920), ..., (989,990)$


So the result I got is $89$. 

However, the result turns out to be $90$. Which one did I miss?

The one you're missing is the pair (899,900).
