Write a function which will assign grades for the courses with respect to the exam score obtained:

Score $90\%$ grade $5$

Score $80\%$ grade $4$

Score $70\%$ grade $3$

Score $60\%$ grade $2$

Score $50\%$ grade $1$

Score $50\%$ grade $0$

Then, using the following piece of code $30+$round$(70*$rand$(20,1))$ generate twenty random score values $($note they will range between $30$ and $100),$ and calculate their respective grades using your function. Calculate the mean grade.

Then plot them as points, in such a way that x axis contains just given score's index $($location in the score vector$),$ and y axis contains the grade value, but$$each of the six grade categories$$is shown with a$$distinct$$marker style. Incluse a horizontal line representing the overall mean grade.

I need the answer in r code please.