BCS Exam ASTQB Topic 15 Question 9 Discussion

You are testing a machine that scores exam papers and assigns grades. Based on the score achieved the grades are as follows:

1-49 = F, 50-59 = D-, 60-69 = D, 70-79 = C, 80-89 = B, 90-100=A

If you apply boundary value analysis, how many test cases will you need to achieve minimum test coverage?