Four fair coins are tossed simultaneously. Find the probability of the random variable X = number of heads and compute the following probabilities:

a) obtaining no heads

b) precisely 1 head

c) at least 1 head

d) not more than 3 heads.

I'm not sure how to set up these probabilities.

For example problems, the book has

for a binomial distribution and

for a symmetric case. There are other probability functions that they list as well.

My biggest problem is finding out the proper way to write these probabilities out, or what probability function can be used (if any).

For example, in a), I can logically see that it would be (1/2)*(1/2)*(1/2)*(1/2), but hibution B(ow can I write this as a probability function?