Answers:
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Work Shown:
AB + BC = AC
3x + x = 20
4x = 20
x = 20/4
x = 5 is one of the answers
AB = 3x = 3*5 = 15 is another answer
BC = 5 is the third answer (since BC = x)
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Check:
AB+BC = 15+5 = 20 = AC
The answers have been confirmed.
See the diagram below.