Answer:
The length of the missing diagonal is 4 miles.
Explanation:
To find the length of the missing diagonal, we can use the formula for the area of a rhombus, which is:

We know that the area is 20 square miles and one of the diagonals is 10 miles, so we can substitute these values into the formula as follows:

Simplifying the equation, we get:

Dividing both sides by 10, we get:

Therefore, the length of the missing diagonal is 4 miles.