Perimeter = 2L + 2W = 400 = 2(L+W)
Therefore L + W = 200
Area = L * W = 9100
Where L= Length and W = Width
Find Multiples of 9,100 which can sum up to 200. The answer is 130 and 70.
Checking.
130 * 70 = 9100
2(130 + 70) = 400
The possible width of the Fountain is 70 feet if the Length is longer or 130 feet if the length is shorter.