Android Database SQLite Average

I am making an app for android with a SQLite Database that have only one table and two columns: one for names and the other for marks. Also, I can see the information of the database in a listview and I can add more elements to it. How can I make the average of the marks which are in the database? And how can I delete a row?

I paste my database helper

public class PersonDatabaseHelper {

private static final String TAG = PersonDatabaseHelper.class.getSimpleName();

// database configuration
// if you want the onUpgrade to run then change the database_version
private static final int DATABASE_VERSION = 1;
private static final String DATABASE_NAME = "mydatabase.db";

// table configuration
private static final String TABLE_NAME = "person_table";         // Table name
private static final String PERSON_TABLE_COLUMN_ID = "_id";     // a column named "_id" is required for cursor
private static final String PERSON_TABLE_COLUMN_NAME = "person_name";
private static final String PERSON_TABLE_COLUMN_PIN = "person_pin";

private DatabaseOpenHelper openHelper;
private SQLiteDatabase database;

// this is a wrapper class. that means, from outside world, anyone will communicate with PersonDatabaseHelper,
// but under the hood actually DatabaseOpenHelper class will perform database CRUD operations
public PersonDatabaseHelper(Context aContext) {

    openHelper = new DatabaseOpenHelper(aContext);
    database = openHelper.getWritableDatabase();
}

public void insertData (String aPersonName, String aPersonPin) {

    // we are using ContentValues to avoid sql format errors

    ContentValues contentValues = new ContentValues();

    contentValues.put(PERSON_TABLE_COLUMN_NAME, aPersonName);
    contentValues.put(PERSON_TABLE_COLUMN_PIN, aPersonPin);

    database.insert(TABLE_NAME, null, contentValues);
}

public Cursor getAllData () {

    String buildSQL = "SELECT * FROM " + TABLE_NAME;

    Log.d(TAG, "getAllData SQL: " + buildSQL);

    return database.rawQuery(buildSQL, null);
}

// this DatabaseOpenHelper class will actually be used to perform database related operation

private class DatabaseOpenHelper extends SQLiteOpenHelper {

    public DatabaseOpenHelper(Context aContext) {
        super(aContext, DATABASE_NAME, null, DATABASE_VERSION);
    }

    @Override
    public void onCreate(SQLiteDatabase sqLiteDatabase) {
        // Create your tables here

        String buildSQL = "CREATE TABLE " + TABLE_NAME + "( " + PERSON_TABLE_COLUMN_ID + " INTEGER PRIMARY KEY, " +
                PERSON_TABLE_COLUMN_NAME + " TEXT, " + PERSON_TABLE_COLUMN_PIN + " TEXT )";

        Log.d(TAG, "onCreate SQL: " + buildSQL);

        sqLiteDatabase.execSQL(buildSQL);
    }

    @Override
    public void onUpgrade(SQLiteDatabase sqLiteDatabase, int oldVersion, int newVersion) {
        // Database schema upgrade code goes here

        String buildSQL = "DROP TABLE IF EXISTS " + TABLE_NAME;

        Log.d(TAG, "onUpgrade SQL: " + buildSQL);

        sqLiteDatabase.execSQL(buildSQL);       // drop previous table

        onCreate(sqLiteDatabase);               // create the table from the beginning
    }
}

}