Question

A poll found that 5% of teenagers (ages 13 to 17) suffer from arachnophobia and are extremely afraid of spiders. At a summer camp there are 11 teenagers sleeping in each tent. Assume that these 11 teenagers are independent of each other. (Round your answers to four decimal places.)

a) Calculate the probability that at least one of them suffers from arachnophobia. - 0.4312
b) Calculate the probability that exactly 2 of them suffer from arachnophobia? 0.08666
c) Calculate the probability that at most 1 of them suffers from arachnophobia? What is C?

Answer 1

a) Calculate the probability that at least one of them suffers from arachnophobia.

x = number of students suffering from arachnophobia

= P(x ≥ 1)

= 1 - P(x = 0)

= 1 - [0.05⁰ x (1 - 0.05)¹¹⁻⁰ ]

= 1 - (0.95)¹¹

= 0.4311999 = 0.4312

b) Calculate the probability that exactly 2 of them suffer from arachnophobia? 0.08666

= P(x = 2)

= (¹¹₂) x (0.05)² x (0.95)⁹

where ¹¹₂ = 11! / (2!9!) = (11 x 10) / (2 x 1) = 55

= 55 x 0.0025 x 0.630249409 = 0.086659293 = 0.0867

c) Calculate the probability that at most 1 of them suffers from arachnophobia?

P(x ≤ 1)

= P(x = 0) + P(x = 1)

= [(¹¹₀) x 0.05⁰ x 0.95¹¹] + [(¹¹₁) x 0.05¹ x 0.95¹⁰]

= (1 x 1 x 0.5688) + (11 x 0.05 x 0.598736939) = 0.5688 + 0.3293 = 0.8981