The calculated value of the missing side length is any value from 10 to 20 (inclusive)
How to determine the missing side length
From the question, we have the following parameters that can be used in our computation:
The triangle
Side lengths = 5 mi and 15 mi
Using the triangle inequality theorem, which states that the sum of any two sides of a triangle is greater than or equal to the third side
Let the third side be c
So, we have
5 + 15 ≥ c
5 + c ≥ 15
15 + c ≥ 5
Evaluate
20 ≥ c
c ≥ 10
c ≥ -10
Ignore the negative result
20 ≥ c
c ≥ 10
Combining the two inequalities, we have
10 ≤ c ≤ 20
Hence, the missing side length is any value from 10 to 20 (inclusive)