Math Probability
posted by Antonio .
If the probability that event A will happen is 1/4 and the probability of A not happening is 3/4, find the probability that event A happens 3 times out of 7 experiments

A binomial distribution applies when
1. Trials have exactly two possible outcomes
2. probabilities remain constant throughout trials.
3. there is a defined number of trials.
4. trials are independent of each other.
5. the random variable is the number of successes.
Since the problem appears to satisfy all the requirements, we will use a binomial distribution to model the probabilities.
p=P(A)=1/4
q=1P(A)=3/4
From (p+q)^7, we look for the term
P(3)
=(7,3)p^3*q^4
=(7!/(3!4!))(1/4)^3(3/4)^4
=2835/16384