Question 1142037
Broth/soup = 1/4.  We are given the soup is 16 lbs., so to get 1/4 broth, that means the broth must be 4 lbs.  (Because broth/soup = 4/16 = 1/4.)
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To find out how much broth we must add to get a soup that is 1/3 broth:
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Broth/soup = (4+x)/(16+x) = 1/3
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Cross-multiply:
3(4+x) = 1(16+x)
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12 + 3x = 16 + x
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2x = 4
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x = 2
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Sarah must add 2 lbs. of chicken broth.  This will give her 6 total lbs. of broth and 18 total lbs. of soup, for a soup that is 1/3 broth.