Let S be the length of the short side of the fence.

Thus, the length of a long side of the fence is 3S.

There are 2 of each side, so the whole fence is 2S + 2 * 3S = 2S + 6S = 8S long.

The whole fence is 8S = 640 feet long.

Thus, the short side of the fence that needs to be replaced is S = 640 / 8 = <<640/8=80>>80 feet long.

# Answer

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A rectangular plot of private property is fenced in by a chain-link fence. The long sides of the plot are three times the length of the short sides. One short side of the fence is rusted from being hit by a sprinkler and needs to be replaced. If all the sides of the fence together are 640 feet long, how many feet of fence need to be replaced?
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80
A rectangular plot of private property is fenced in by a chain-link fence. The long sides of the plot are three times the length of the short sides. One short side of the fence is rusted from being hit by a sprinkler and needs to be replaced. If all the sides of the fence together are 640 feet long, how many feet of fence need to be replaced?
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