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November 2nd, 2017, 03:06 PM  #1 
Newbie Joined: Nov 2017 From: rasht iran Posts: 4 Thanks: 0  how we can guess a last number from a 4number?
hello guys. always i has hate from math but a persion asked a puzzle from me! i can not understand he how can guess this ! for more pls read mt next... ( sorry for poor english  i hope you undestand me) he told me write a 4number like 4321 and next write from back 1234 now do 4321  1234 and find answer. 43211234= 3087 he told me now say me 3 first number from your answer. i told 308 and he told its 3087!!! for new example: i did 9345 5439= 3906 i and say 390* . but he know what is * !!!! he guess * is 6 !!!! but he with what algorithm can find it ? with what algorithm i can find number of 4 fom a 4number?  thanks for your help, i am wait for give a good answer from you. Last edited by edrisdolati; November 2nd, 2017 at 03:09 PM. 
November 2nd, 2017, 04:44 PM  #2 
Senior Member Joined: Sep 2015 From: USA Posts: 2,099 Thanks: 1093 
what would you do if the reversed number is larger than the original?

November 3rd, 2017, 12:45 AM  #3 
Newbie Joined: Nov 2017 From: rasht iran Posts: 4 Thanks: 0  
November 3rd, 2017, 12:46 AM  #4 
Newbie Joined: Nov 2017 From: rasht iran Posts: 4 Thanks: 0  
November 3rd, 2017, 03:52 AM  #5 
Newbie Joined: Mar 2016 From: United Kingdom Posts: 2 Thanks: 1 
Your friend is using the idea of Modulus (to base 9). 4321 mod 9 = 1 (4321/9 = 480 rem 1). 4 + 3 + 2 + 1 = 10; 10/9 = 1 rem 1. 1234 mod 9 = 1 (1234/9 = 137 rem 1). Also 1 + 2 + 3 + 4 = 10; 10/9 = 1 rem 1. The difference between the number chosen and that found by reversing its digits will always have modulus 0 to base 9 ie divisible by 9. So, knowing three digits, your friend finds a fourth that brings the total to a multiple of 9. 3 + 0 + 8 = 11, needs 7 to bring it to 18. 3 + 9 + 0 = 12, needs 6 to bring it to 18. 
November 3rd, 2017, 04:52 AM  #6 
Newbie Joined: Nov 2017 From: rasht iran Posts: 4 Thanks: 0 
hmmmm game with math jofo dear friend thanks for your help. thanks 

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4number, guess, number 
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