If you liked the Fun Fact Multiplication by 11, you’ll enjoy seeing how to take that idea one step farther. Here’s a quick way to multiply 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:
23×111= 2553
41×111= 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
57×111= 6327
because 5+7=12, but then you have to carry the one twice.
If the number you are multiplying by 111 is a three-digit number, say ABC, then the answer will have five digits (though it may be six if there is a carry involved):
the first digit is A,
the second digit is A+B,
the third digit is A+B+C,
the fourth digit is B+C,
the fifth digit is C.
Again, you must remember to carry if any of these sums is more than one digit. Thus 123×111=13653, 241×111=26751, and for an example where carrying is needed, 352×111=39072. (Because of the carries, it may be easier to do the sums and write the answer down from right to left.)
Presentation Suggestions:
Do the Multiplication by 11 Fun Fact first.
The Math Behind the Fact:
Multiply using the traditional (long) method for multiplication, and you will find that the above shortcut works because it is doing exactly the same sums that you would have to do using the traditional method for multiplication.
See some of the other Fun Facts on lightning arithmetic.
How to Cite this Page:
Su, Francis E., et al. “Multiplication by 111.” Math Fun Facts. <https://www.math.hmc.edu/funfacts>.
References:
A. Benjamin and M. Shermer, Secrets of Mental Math, Three Rivers Press, 2006.
Fun Fact suggested by:
Francis Su