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Prime Ponderings

Math brain teasers require computations to solve.

 

Puzzle ID:#19862
Fun:*** (2.71)
Difficulty:*** (2.78)
Category:Math
Submitted By:snaps*au****

 

 

 



What are the largest and smallest 5-digit numbers that satisfy the following conditions?

1. Each digit of the number is a prime digit.
2. Each successive pair of digits forms a 2-digit number that is NOT a prime number.
3. Each of the prime digits must appear at least once in the 5-digit number.

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Answer

Largest - 35772
Smallest - 32257

Prime digits are 2, 3, 5 and 7. Taken as pairs, the only combinations that fit criterion 2 are:
22, 25, 27, 32, 33, 35, 52, 55, 57, 72, 75 and 77.
Of that list, 33 is the only number that contains a 3 as the second digit and so for criterion 3 to be satisfied, 3 can only appear at the start of the 5-digit numbers. From there it is simply a matter of choosing the next digits using the list above so that criterion 2 is satisfied, making sure that criterion 3 is also satisfied.

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