I got (c-frac{1}{a}) =1 by rearranging (b=1-frac{1}{c}) to get a value for c ( assuming b is not equal to 1) and then (assuming a not equal to zero) subbing in the value for c and a into (c-frac{1}{a}) and rearranging.
Can we ask where the question came from/why you are interested in it?
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