The probabilities of a student getting grade A, B, C and D are 0.2, 0.3, 0.15 and 0.35 respectively. Then the probability that a student gets at least C grade is:
0.65
Probability that a student gets atleast C grade = (Probability of getting grade A + Probability of getting grade B + Probability of getting grade C)
= 0.2 + 0.3 + 0.15 = 0.65