Bunuel
A rectangular public park has an area of 3,600 square feet. It is surrounded on three sides by a chain link fence. If the entire length of the fence measures 180 feet, how many feet long could the unfenced side of the rectangular park be?
I. 60
II. 90
III. 120
A. I only
B. II only
C. III only
D. I and III only
E. I, II and III
We have the following equations:
3,600 = WL
Since 3 sides of the fence add to 180, we have:
2L + W = 180
W = 180 - 2L
Substituting, we have:
3,600 = L(180 - 2L)
3,600 = 180L - 2L^2
1,800 = 90L - L^2
L^2 - 90L + 1800 = 0
(L - 60)(L - 30) = 0
L = 60 or L = 30
Thus, we see that if L = 60, then W = 180 - 2(60) = 60, OR if L = 30, then W = 180 - 2(30) = 120.
Since the length of the unfenced side of the park is equal to W, it could be 60 OR 120.
Alternate Solution:
We have the following equations:
3,600 = WL
2L + W = 180
Notice that according to the last equation, the unfenced side is W.
Let’s test each Roman numeral:
Roman numeral I: W = 60
If W = 60, then 2L = 180 - 60 = 120 and thus, L = 60. Since WL = 60 x 60 = 3600, we see that W = 60 is possible.
Roman numeral II: W = 90
If W = 90, then 2L = 180 - 90 = 90 and thus, L = 45. Since WL = 90 x 45 = 4050, we see that W = 90 is not possible.
Roman numeral III: W = 120
If W = 120, then 2L = 180 - 120 = 60 and thus L = 30. Since WL = 120 x 30 = 3600, we see that W = 120 is possible.
Answer: D