Take a $2$ digit number, then reverse it. Take the smaller number away from the larger.  Examples:  $26:$(reversed becomes $62$)$62-26=36$  $71:$(reversed becomes $17$)$71-17=54$  Try some more for yourself.  What do you notice about the answers? Is it always true? Can you tell why? 
Overseas Competition->Knowledge Point->Combinatorics->Strategies and Operations->Operational Problem->Number Operation