why is the carbon with methyl group and hydrogen attached R here?
H is 4th priority. Me is 3th priority. Carbon with 2 methyl groups attached is 1st. --> C(C,C,C) mass number 3C= 36 Carbon with OH and H is 2nd --> C(C,O,H) mass number C+O+H= 29
so we get anticlockwise rotation which must be S.