I don't know where the 8 comes from. It's a bit mysterious to me. You will have to look at the textbook and/or lecture notes to help me there. Perhaps someone else sees where the 8 comes in. But their result is correct. It's just weird to me. I would see \$\frac{10}{\sqrt{2}}\$ for the RMS voltage. Then \$\frac{15}{10+15}\$ as a divider. Then finally \$\frac{5}{5+26}\$ as the last divider. That gets to the same place. But the 8, and their splitting of 25 into
5*5 just seems mysterious. Your question is good. I must be blind to something important. So I'll +1 the question.