Perimeter of a rectangle is: length plus width plus length plus width
Which is (length + width) * 2
320÷2 equals 160
160 m represents the sum of the ratio which is 8 (5 + 8)
You used to cross multiplying method :
160 m = 8
 ? 5
160 X 5
_______
 8
= 800
 ___
 8
= 100 m is the Length 
You do the same cross multiplying method and you get : 
160 x 3 
______
 8
= 60 m for the width