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.
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
