1. ## Probability Question

I do not understand this problem even after seeing the solution to the problem.

the part that confuses me the most is why P[3 don't respond] equals to that when the answer is 4x

2. ## Re: Probability Question

Do you understand that $P[\text{respond}]=0.7$ ?

At least 3 never respond is the same situation as at most 1 responds.

So we have a binomial distribution to evaluate at $k=0,1$

$P[\text{at least 3 don't respond}] = P[\text{0 or 1 respond}] = (1-0.7)^4 + \dbinom{4}{1}(0.7)^1 (1-0.7)^3 = (0.3)^4 + 4 (0.3)^3(0.7)$

This is choice A.