Question

If a seed is planted, it has a 80% chance of growing into a healthy plant. if 9 seeds are planted, what is the probability that exactly 4 don't grow?

Answer 1

The probability of a seed not growing into a healthy plant is 1 - 0.8 = 0.2.
$P(4\ seeds\ don't\ grow)=9C4\times0.2^{4}\times0.8^{5}=0.066$
The answer is the probability = 0.66 or 6.6%

Answer 2

This is a binomial probability because the plant can grow healthy or not. You should apply binomial probability formula when x = 4, n = 9, p = 0.2 (because one wants to know the chance of no growth).
P(x=4)= 9C4*0.2^4*0.8^(9-4) = 9!/(4!(9-4)!)*0.2^4*0.8^5 =126*0.2^4*0.8^5=0.066 = 6.6%