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If we consider 100 units of work as total work. (To avoid fractions :3)

Then in 1 hour one machine does (100/4) = 25 units of work, while another one does (100/2)=50 units of work.

They together do 75 units of work in 1 hour ,so 100 units of work is done in (100/75) hours or 4/3 hours

So, **(C)4/3 is the answer**