Hi Seulki,

This is a binomial distribution problem. There is an experiment, tossing a coin, which is repeated 30 times and the probability of tossing a head is 1/2 on each of the 30 trials. Thus for example the probability that the head shows up more than 12 times is

I am not sure how you are to evaluate this. You can find each one of the terms and add them, you can use a binomial table or you can use the normal approximation to the binomial.

Harley