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neves dda tsuj (Posted on 2004-02-22) Difficulty: 3 of 5
You have a simple (base-ten, whole number) calculator which can perform only two operations: visually reversing a number, and adding seven.

Prove that you can use this calcluator to convert any number to 1.

Notation: use ~ to denote reversal, as in
~53 = 35

No Solution Yet Submitted by DJ    
Rating: 4.5000 (6 votes)

Comments: ( Back to comment list | You must be logged in to post comments.)
Some Thoughts re: Straightforward , but not so "executable" Solution | Comment 9 of 11 |
(In reply to Straightforward Solution by Richard)


Both your procedure and mine work - no doubt, but the number of operations needed is totally different .
In your case this number is proportional to the initial number and in mine- to the number of digits.
Consider a one digit number 5 . You really want to press
"add 7" (and all you have is a simple calculator) 14000 times to get 10^5- which is 5 mod7 -and then to inverse it???. Please compare it with 5==>40==> 4 ==> 60==> 6 ==>20 ==>2 ==>30==> 3 ==>10==> 1 about 25 operations!!

I agree that this consideration was not part of the question but still let us give preference to "executable" solutions.

Edited on February 24, 2004, 11:37 am
  Posted by Ady TZIDON on 2004-02-24 11:35:50

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