Write a program that has variables to hold three test scores. The program should ask the user to enter three test scores and then assign the values entered to the variables. The program should display the average of the test scores and the letter grade that is assigned for the test score average. Use the grading scheme in the following table: 

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Test Score Average 90-100 80-89 70-79 60-69 Below 60 Letter Grade A B C D F