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Erase a prime (Posted on 2020-08-03) Difficulty: 2 of 5
Write down the first n primes.

Erase one of them and the mean of the remaining numbers is 10.

What number did you erase?

No Solution Yet Submitted by Jer    
Rating: 3.0000 (1 votes)

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Instant solution | Comment 3 of 4 |

 When the puzzle was on the reviewing board I saw that it is a d1  puzzle since the difference between a(n) and n-1 must be divisible
                    by 10 
.So visual scan of both series will yield the solution

                 ....Write down  n-1=     0,1, 2,3,4,5,6,7,8,..11,12

                                              ....,77 ,58 ,41  ,2,5,10,17,28  and 

   scanning from left to right, look for identical digits 
of the odd numbers  in both series.   Check the result.                                                                      l
 
                                                         answer n=8     n-1=7    
.          10  =    7 / (77-7)

number 7, the 4th prime was erased    



Edited on August 4, 2020, 9:13 am
  Posted by Ady TZIDON on 2020-08-04 07:55:54

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