Trevor Green

1. This is a numbers-only keypad. So let's think about numbers.

2. Numbers that are in their prime.

3. What prime numbers are there to be found on the keypad?

4. The following single-digit numbers are prime: 2, 3, 5 and 7.

5. Now what would the youngest of them be?

6. The order should be: second smallest number, then the largest, then the smallest, then the second largest.

(SOLUTION FOLLOWS)

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7. The answer is 3725.