Calculate total checkboxes ticked and perform calculation (Android Java)

I have checkboxes created dynamically using shared preferences. The label for each checkbox is stored in array.xml. How can I calculate the total number of checkboxes that are ticked and store the total in a variable and further use it for another calculation -- (total/totalCheckboxes)*100?

Here's a snippet of the Java class:-

ListView myList;
Button getChoice, clearAll, button1;
SharedPreferences sharedpreferences;
public static final String MyPREFERENCES = "MyUserChoice";
ArrayList<String> selectedItems = new ArrayList<String>();

@Override
protected void onCreate(Bundle savedInstanceState) {
    // TODO Auto-generated method stub
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    myList = (ListView) findViewById(R.id.list);
    getChoice = (Button) findViewById(R.id.getchoice);
    clearAll = (Button) findViewById(R.id.clearall);
    button1 = (Button) findViewById(R.id.button1);

    ArrayAdapter<String> adapter = new ArrayAdapter<String>(this,
            android.R.layout.simple_list_item_multiple_choice,
            getResources().getStringArray(R.array.Questionnaire));
    myList.setChoiceMode(ListView.CHOICE_MODE_MULTIPLE);
    myList.setAdapter(adapter);
    sharedpreferences = getSharedPreferences(MyPREFERENCES,
            Context.MODE_PRIVATE);
    if (sharedpreferences.contains(MyPREFERENCES)) {
        LoadSelections();
    }

and the array.xml:-

<?xml version="1.0" encoding="utf-8"?>
<resources>

    <string-array name="Questionnaire">

        <item>abc</item>
        <item>jkm</item>
        <item>xyz</item>
        <item>abc2</item>
        <item>jkm2</item>
        <item>xyz2</item>
        <item>abc3</item>
        <item>jkm3</item>
        <item>xyz3</item>

</resources>

Thanks in advance!