I pay a picture to the classmate above. ABC stands for the first question, the second question and the third question respectively.

a+b+c+d+e+f+g=25，①

b+f=2(c+f)，②

a=d+e+g+ 1，③

a=b+c.④

② Substituting ① to get A+2B-C+D+E+G = 25, ⑤

③ Substitute it into ⑤ to get 2b-c+2d+2e+2g = 24, ⑥.

④ Substitute it into ⑤ to get 3b+d+e+g = 25, ⑦.

⑦×2-⑥ 4 b+c = 26。

Because c≥0 and b≤6/ 1/2.

Use ②, ⑧ to eliminate C and get f=b-2(26-4b)=9b-52.

Because f≥0 and b≥5/7/9.

So b=6, that is, six people can only solve the second problem.