Probability of students passing med school

I worked out the following problems though i'm not 100% sure that i've figured them correctly.

62. at Med University, the failure rate is 0.2 per year. There are 15 students registered in the first-year class. Give all your answers correct to 4 decimal places.

a. what is the probability that all of them will pass?

b. what is the probability that exactly 12 of them will pass?

c. what is the probability that at least 12 of them will pass?

a. p = 0.2^0(1-0.2)^15

= 0.8^15

= 0.0352

therefore the probability of all of them passing is 0.0352 or 3.52%

b. p = 0.2^12(1-0.2)^15-12

= 0.000000004(0.8)^3

= 0.000000004(0.512)

= 0.000000002

therefore, the probability of exactly 12 of them passing is 0.000000002

c. p1 = 0.000000002

p2 = 0.2^13(1-0.2)^15-13

= 5.243x10^-10

p3 = 0.2^14(1-0.2)^15-14

= 1.311x10^-10

p4 = 0.0352

now for this question i'm not too sure where to head from here to get the finaly probabilty and not to have 4 separate probabilities.