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Posted by on Saturday, April 2, 2011 at 8:09pm.

A pair of emirps consists of two prime numbers such that reversing the digits of one number gives the other. How many pairs of two-digit emirps exist such that each number in the pair is greater than 11?

  • algebra - , Sunday, April 3, 2011 at 8:41am

    The list of prime numbers between 11 and 100 are:

    11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73 79 83 89 97

    So try them one at a time, say
    number reversed
    11 11 OK
    13 31 OK
    17 71 OK
    19 91 is composite [pair rejected]
    23 32 [rejected]
    ...
    97 79 OK

    So you'll get a list in no time.

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