Round 4.12 to nearest tenth

Ihave trouble finding the nth term of a linear sequence

Yuto solved the equation below. what is the solution to yuto’s equation?

The perimeter of ? cde is 55 cm. a rhombus dmfn is inscribed in this triangle so that vertices m, f, and n lie on the sides cd , ce , and de respectively. find cd and de if cf=8 cm and ef=12 cm. the answer is not cd=20 and de=15. i have tried that one, it does not work.