7. Of all the four digits, how many digits are there when the sum of the first two digits and the sum of the last two digits equals 8?

The solution to this problem: first of all, how many are there whose sum of two digits is eight? 08, 17, 26, 35, 44, 53, 62, 7 1 80, and the four digits consist of two digits. There are 64 hypotheses after matching, but the possibility of starting with 08 should be ruled out, so there are 64-8=56 hypotheses.

Details: 1708,17, 1726, 1735, 1744, 1753,/kloc-0.