Given, the probability that a student will get A, B, C or D grade are 0.4, 0.35, 0.15 and 0.1 respectively.

Events of getting A, B, C or D are mutually exclusive.

1. P(B or C grade) = P(B grade) + P(C grade)

                         = 0.35 + 0.15

                         = 0.50

2. At most C grade means C or D grade

Now,

P(At most C grade) = P(C or D grade)

                           = P(C grade) + P(D grade)

                           = 0.15 + 0.1

                           = 0.25