Thread: Probability Teaser! Binomial dist or what?

Originally Posted by isuckatprobability

Problem: A material is known for 20% breakage under stress. Five different samples of such materials are tested.
A. What is the probability that the third sample resists the stress?

Mr F says: 4/5

B. What is the probability that the third and fourth samples resist the stress?

Mr F says: (4/5)^2

I don't know how to approach this question. I see that 1 material are damaged for every 5 tested, and I was thinking of using the binomial distribution for it. But I don't think I can, because I believe that distribution involves multiple. Can someone give me some direction to go with this?

Here's a question to help you understand the answers:

A coin is tossed 5 times.

A. What's the probability that it comes up heads on the third toss?

B. What's the probability that it comes up heads on the third and fourth toss?

thankyou. That does make sense. So simple, I don't believe!