Lab command line arguments and inputs

Hi, regarding last question in lab of arethmetic operations(calculcating average).
the below script(copied from the answers in the var/assets folder) does not retrive the average,
can someone kindly asssit?
when i type: bob@caleston-lp10:~$ ./calculate-average.sh 1 2 5
nothing is returned.

num1=$1
num2=$2
num3=3 sum=(( num1 + num2 + num3 ))
average=$(echo “$sum / 3” | bc -l)

Check the following steps:
bob@caleston-lp10:~ cp /tmp/assets/calculate-average-answer.sh . bob@caleston-lp10:~ mv calculate-average-answer.sh calculate-average.sh
bob@caleston-lp10:~$ sh calculate-average.sh 1 2 1
1.33333333333333333333

Then chech answer.