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A palindrome is a sequence of digits which reads the same backward or forward. For example, 7447, 1001 are palindromes, but 7455, 1201 are not palindromes. How many 8 digit prime palindromes are there?
asked in Combinatory by (189 points) 1 6 12
retagged by | 229 views

8 digit palindrome = 9*10*10*10 = 9000
but how to get 8 digit prime palindromes ?

5 Answers

+8 votes
Best answer
I think the answer should be 0, as any even digit palindrome(other than 11) cannot be prime. Even digit palindromes will always be divisible by 11(you can check the divisibility test by 11).
answered by (171 points) 6
selected by
A number is divisible by 11. When the difference between the sum of its alternating digits is divisible by 11 (scanned from left to right completely).
For Ex:

2772 has the alternating sum of digits 2-7+7-2 = 0. Since 0 is divisible by 11, so is 2772.

Similarly, for 31415, the alternating sum of digits is 3-1+4-1+5 = 10. This is not divisible by 11, so 31415 is not divisible by 11.
0 votes
i think answer will  be 9*10^3   bcz

9*10*10*10*1*1*1*1 =9000  and each no in multiplication shows the digit place
answered by Loyal (4.4k points) 4 16 51

In the above question, we have to find number of 8 digit prime palindromes but your answer includes both prime and non-prime palindromes. 

For ex- 20000002 
Is it a prime palindrome. ??

0 votes

Let the digits of our 8-digit palindrome n be d1d2d3d4d4d3d2d1. Then the palindrome has the form

n=10000001⋅d1+1000010⋅d2+100100⋅d3+11000⋅d4.

Such n cannot be prime because gcd(10000001,1000010,100100,11000)=11.

So there are NO 8-digit prime palindromes. (However, there are prime palindromes with an odd number of digits, e.g. 101101, 1030110301, 9868998689, 98010899801089.)

The correct answer is 0.

answered by Boss (8.8k points) 3 8 12
what had been the answer if 9 digits prime palindrome was given?
0 votes
answered by Boss (5.8k points) 2 6 15
0 votes
there are  8 digits but the no. should be a prime and palindrome . so first place can be filled by only 5 prime digits (1,3,5,7,9) and second ,third , fourth each digit can be filled by 10 ways .  last four digits will be a mirror image of first four digits.   so total numbers possible = 5*10*10*10= 5000
answered by (365 points) 1 2 8


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