In the following diagram line CF is parallel to line CD. Segment DE has a length that is 4 inches greater than the length of CD. If FG= 15 inches and FE=36 inches then determine the length of CD

Answer: CD = 10

Identify the given information. 
FG = 15
GE = 21
FE = 36
CD = x
DE = x + 4

Form your equation. Due to the Side-Splitter Theorem, FG/GE = CD/DE.
15/21 = x/x + 4

Cross-multiply and solve.
15(x + 4) = 21x            Distributive Property
15x + 60 = 21x           Subtract 15x from both sides
60 = 6x                      Divide by 6 to both sides
x = 10                         Answer!