Question

The perimeter of the figure is 20 inches. What is the missing side length?

Answer 1

When finding perimeter you add all the sides up.
So if you're looking for the perimeter, which is 20, just add 8 + 4 + 4
That equals 16.
So do 20 - 16.
Then you get your answer which is 4 in

You could also do:
20-8
That's 12
Then 12-4
That's 8
Then do 8-4
To get you your answer of 4 in

Answer 2

You would do the whole perimeter (the length of all the sides added up) minus the three known sides to find the missing side.
20 - 4 - 4 - 8 = 4, so the answer would be 4 inches.