Can someone help with a question involving probability and the binomial distribution formula

Question:
A multiple choice test has 15 questions, each with 4 possible answers only one of which are correct. Suppose a student guesses the answers to all 15 questions. This may be viewed as a binomial experiment with 15 trials and a probability of success of 1/4 or 0.25 (where success is defined as marking the correct answer). For every binomial distribution, we must state the number of trials and probability success. There is no need to state the probability of failure.

a) Write the formula of the binomial distribution with the number of trials n=15 and probability of success p=0.25. State the mean and variance of the binomial distribution.

b) Calculate the value of p(0), the probability of marking none of the 15 answers correctly, using the binomial distribution formula in part (a).

---------------

I did part (a) and found the mean=3.75 and the variance=2.8125. I'm just not sure how to do part (b). Any help is appreciated

a) P(S = s) => 15Cs x (0.25)^s x (0.75)^(15 - s)

Mean = 15 x 0.25 = 3.75

Variance = 15 x 0.25 x 0.75 = 2.8125

b) P(S = 0) => 15C0 x (0.25)^0 x (0.75)^15 = 0.75^15

i.e. 0.013

:)>