A retreat longwall coal mining system utilises twin accesses on both the intake and return sides of a panel whose dace width is $300$m$.$ if the access drives are $5$m wide and chain pillars are $20$m wide between the accesses determine the extraction ratio for the following cases a$)$ Panel under development b$)$ Panel under production It may be assumed that rib pillars $40$m wide are left for support between panels. It can also be assumed that a barrier pillar of $40$m width is left to protect the main mine entries and that the panel length is $3$km