Question

Find the number of three-digit numbers from $100$ to $999$ including all numbers which have any one digit that is the average of the other two.

Answer 1

Let $A=100,B=999$ be the whole numbers there are between $A$ and $B$ ,$B$ be the bigger number.

Then, bigger number $=B-A+1$

$=999-100+1=898$

Hence, this is the answer.