Grade book project

I have this prompt to do with javascript and I cant really seem to get a hand on how to do it and everywhere I've searched hasn't been specific enough so I was hoping posting the prompt could get me some better more specific help. Thanks to anyone who is willing to give me a hand!

This project is designed to prove you can use Javascript programming on your own. Using a website called JS.do you can write some Javascript code online. The website should allow you to create an account and let you save your code. If this becomes a problem then you should copy your code and save it to a document in Google Drive.
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Project Description:
This project is called Grade Book, for it you will have to create an array and then calculate the average.
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Step 1. You will need to create an array and give it at least 10 values.
Step 2. You will need to create a loop to add up all of these values into a variable called sum.
Step 3. After the loop runs there should be a line which writes out the average
Step 4. Finally, using an if statement(s) or switch statement(s) tell me what the letter corresponding to the grade average would be
A- Over 90
B- 80-89
C- 70-79
D- 64-69
F- Under 64
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So an example would be if the average is 75 the output should look like this:
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The class average is 75
The average grade is a C
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Hints
The average is calculated by the sum/number of values (10 in this case)
For JS.do you will have to use Document.write instead of console.log in order to print text
There are some sample projects on the JS.do site in case you need to check how to do something. You can also refer back to Code Academy