What are the -OCOR group on the right side 4th line of 346th page and -COR group on the left side 19th page of the chemistry 11th STD part2 text book called? Are they same ?

'x' gm of a compound A3B2C5 contains 'y' gm of A atoms

 

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how to represent the possible fischer projections for 2-bromohexane