If one more octagon was added, the perimeter would be 29. The screenshot below shows the relationship between the perimeter and the number of octagons added.
Work Step by Step
The perimeter could be founded using the function P(n)=7(n)+8 , where n represents the number of octagons added to 1 octagon as when we only have 1 octagon and 0 octagons is added, the P(0)=7(0)+8 = 8. Therefore, when two octagons are added the perimeter for 3 octagons is P(2)=7(2)+8=22. Thus if one more pentagon is added to make 4 octagons the perimeter would be P(3)=7(3)+8=21+8=29.