# An allergist claims that 50% of the patients she tested are allergic to some type of weed. What is the probability that exactly 3 of her next 4 patients are allergic to weeds?

## What is he probability that none of her next 4 patients are allergic to weeds?

$.25$
Since the allergist wants to know the probability of exactly $3$ and that there's a set number of trials with a given probability, we use $b p \mathrm{df}$. The probability is $.50$, the number of trials is $4$, and the x value is $3$:
$b p \mathrm{df} \left(4 , .5 , 3\right) = .25$