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inverse the equation:
(c-b) / (c-a) = 1/2
Then solve for c in terms of a and b
c-a = 2c-2b
c = 2b-a
Plug in c for the inversed equation
(2b-a-b) / (c-a) = 1/2
(b-a) / (c-a) = 1/2
(5b-5a) / (c-a) = 5/2 = 2.5