The answer is 247. You can do this. You set the hundred digits as A, the ten digits as B and the single digits as C. Then, from the meaning in the question, we can know that B = 2A and C = A+5. The first condition c must be radix, so a must be even. A # 0, a, b and c are all natural numbers greater than 1 less than 9. C=a+5.a may be 2 or 4. If a=2, then 2+4+7= 13 is a prime number, and a=4, 4+8+9=2 1 is not a prime number, so a can only be 2, so you can calculate it yourself!