Ok... Perimeter is = 2l + 2w
w being width and l being length.
We know that perimeter is equal to 400 so
400 = 2l + 2w
The length is 15 meters less than the width, so in notations, l = w - 15
We can substitute than in the equation to get 2(w-15) + 2w = 400
Expanding will give 2w - 30 + 2w = 400
Solving w gives w= 430/4 = 107.5 meters
Substituting in original equation for w to find l, 400 = 2l + 2(107.5)
400 = 2l + 215
2l = 185
l = 92.5 cm
Or you can just use the question to know that the length is 15 times less than the width, so 107.5 - 15 = 92.5
Length = 92.5 cm
Width = 107.5 cm