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I'm having trouble with coming up with a minimal solution to this problem: create a 8 bit adder/sub using only one 4 bit ripple carry adder and muxes. This is what I'm thinking but I don't know how to make it any bit simpler:

We can calculate the sum of first 4 bits using the ripple carry adder and then calculate the sum of the next 4 bits(bits 4-7) using full adders built with mux's. A full adder needs 2 muxes(one for sum output and another for carry out) and this means I need to use 8 mux's to calculate the sum of bits 4-7.....any hints on how to improve on my idea or how to make this design more simpler? Thanks