Omar is a civil engineering employee working at XYZ road Construction Company. He earns a salary of $24000$ Omani Riyals annually. His salary increase by $50$ Omani Riyals every year.

a$)$ Calculate his annual salary for the $5$ th year.

b$)$ The total amount he would earn in $6$ years.

c$)$ How many years will it take him to reach an annual salary of $84000.$

d$)$ If Omar enjoys annual increment of $50$ Omani Riyals for the first $4$ years after which his increment goes up by $30$ Omani Riyals. Calculate his annual salary in the $8$ th year.