Bash doesn't directly support such operation for Floating-point numbers. You'll need external tools such as awk, bc to get the job done. I would rather keep the code simple in such cases.

read n
sum=0
for ((i=0;i<$n;i++))
do
    read temp
    sum=$(awk "BEGIN {print $sum+$temp; exit}")
done
printf "%.3f\n" $(bc -l <<< "$sum/$n")