Please help I don't understand 20 POINTS

Multiply. write your answer in simplest form. 3/8 x 5/7

Find the value of the combination. 5 c 5

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.