Since triangles DEF and GEH are similar, we can use the properties of similar triangles to find the length of side FE.
From the information given:
- Side DF in triangle DEF is 10.
- Side GH in triangle GEH is 4.
- Side EH in triangle GEH is 8.
The sides of similar triangles are proportional. Therefore, we can set up a proportion based on the corresponding sides:
\[ \frac{DF}{GH} = \frac{FE}{EH} \]
Substituting the known values into the proportion gives us:
\[ \frac{10}{4} = \frac{FE}{8} \]
Now, we can cross-multiply to solve for FE:
\[ 10 \cdot 8 = 4 \cdot FE \]
This simplifies to:
\[ 80 = 4 \cdot FE \]
Dividing both sides by 4 gives:
\[ FE = \frac{80}{4} = 20 \]
Thus, the length of side FE is 20.