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Amy calculate $47\times86$ wrong. Her teacher says her result which has a difference of $172$ from the correct one. After checking, Amy finds that she writes one of the four digits two multipliers the result as $9$. Which digits does she write incorrectly? 
Overseas Competition->Knowledge Point->Combinatorics->Number Puzzles
The difference between the wrong answer and the correct one is $172$. $172\div86=2$, $172\div47=3R31$.That means Amy writes neither $ 8$ nor $6$ wrong. Then, compare $49\times86$ and $97\times86$, so we can get the correct answer. $49\times86-47\times86=172.$ 