Write down the first n primes.
Erase one of them and the mean of the remaining numbers is 10.
What number did you erase?
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
.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