Answer:
(c) 6.25
Explanation:
You want the value of x = FG, given similar triangles DFG and RPQ.
Proportion
Corresponding sides of similar polygons are proportional. This means ...
FG/PQ = DF/RP
x/5 = 10/8
x = 5(10/8) = 50/8 . . . . multiply by 5
x = 6.25
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Additional comment
It can be a bit confusing that x is the middle-length value. That is ...
4 : 5 : 8 = 5 : x : 10
The numbers in the second ratio of shortest-to-longest sides are 5/4 = 1.25 times those in the first ratio. Then x=1.25·5 = 6.25.