We'll check all the integers between 20 and 30:
21 is not prime because it is divisible by 3 and 7.
22 is not prime because it is divisible by 2 and 11.
23 is prime but its digits do not differ by 7 because 3-2=1 not 7.
24 is not prime because it is divisible by 2,3,4,6,8 and 12.
25 is not prime because it is divisible by 5.
26 is not prime because it is divisible by 2 and 13.
27 is not prime because it is divisible by 3 and 9.
28 is not prime because it is divisible by 2,7, and 14.
29 is prime and its digits DO differ by 7 since 9-2=7.
So that's it! 29
Edwin
