电子表格中,怎样运用函数公式去求及格率和优秀率?

及格率：设：a1=66,a2=59,a3=80,a4=30,a5=90,
a6=if(countif(a1:a5,">=60")/counta(a1:A5)=0.6,即60%
优秀率：设：a1=A,a2=B,a3=C,a4=D,a5=A
a6=if(countif(a1:a5,"A")/counta(a1:A5)=0.4,即40%