trick to multiply a two-digit number by 111

To multiply a two-digit number by 111, add the two digits and if the sum is a single digit, write this digit TWO TIMES in between the original digits of the number. Some examples:
23x111= 2553 
41x111= 4551
The same idea works if the sum of the two digits is not a single digit, but you should write down the last digit of the sum twice, but remember to carry if needed. So
57x111= 6327
because 5+7=12, but then you have to carry the one twice.


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