The first square has side lengths of 4 units(because the square root of 16 is 4). The second square has side lengths of 8 units. The third square has side lengths of 12 units. As you can tell, the side lengths increase by 4. So the fourth square will have side lengths of 16 units, and the fifth square will have side lengths of 20 units. The question wants the perimeter, so the answer is 20*4(sum of 4 sides of equal lengths) which is 80 units.