Saturday, 12 October 2013

divisibility - How to divide by 12 quickly?



Let nN be divisible by 12 and n/12<100. Is there a way of computing n/12 rather quickly using mental arithmetic (e.g. for 972/12, 1044/12, etc.)?



For example, the number 11 seems to have a nice property. When we consider 836/11=d then
770<836, but 880>836, so the first digit of d must be 7. And since the last digit of 836 is 6, so is the last digit of d. This gives us d=76.
Or consider 693/11=d. Then 660<693 (and 770>693), so the first digit of d must be 6 and the second is 3 since this is the last digit of 693. This gives us d=63.



Now, is there another (possibly similar) approach for dividing by 12 (or even 13, 14, etc.)? But I am mostly interested in a "trick" for the number 12 (just using mental arithmetic).



Answer



One can memorize multiples of 12 that are less than 100. Then if n/12=10a+b you can guess a quite fast. And there are two options for b. Now if n=100a+(2a+b)10+2b then you can decide about b by comparing 2a+b and the second least significant digit of n.


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