Answer:
(I'll leave cm out to save space and time in the answer)
the perimeter is 500, there are 5 sides. so on average a side will be 100.
that's already the important key idea.
it's indeed 100 for the third side, the second will be 10 less, the fourth 10 more (making the 2and and the 4th side 200 in sum, and therefore still 100 on average).
repeat this for the 1. and 5. side and we get:
80
90
100
110
120
