Any Prime number greater than 10 is an odd number but cannot end in 5 as this makes it divisible by 5.
Therefore the ones digit of any prime number greater than 10 must be 1, 3, 7 or 9.
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Prime numbers greater than 10 will only end in 1, 3, 7 or 9.
The prime number between 20 and 30 whose ones digit is 7 greater than its tens digit is 23. The tens digit is 2, and the ones digit is 3, which is 7 greater than 2. Additionally, 23 is a prime number because it is only divisible by 1 and itself, with no other factors.
The only other single-digit number higher than 5 which is a prime number is 7 - 9 is equal to 3 squared, and is thus not a prime number.
23
679 is the product of the largest single-digit prime number and the largest two-digit prime number.