# Thread: If x, what is prob will be first in 3 or more tries (conditional prob)

1. ## If x, what is prob will be first in 3 or more tries (conditional prob)

I can do (i) and (ii) but stuck on (iii) and (iv)

Every year 2 teams, the ramblers and the strollers, meet for a quiz night. From past results it seems that in years where the ramblers win, prob of them winning next year is 0.7 and in years when strollers win, prob of them winning is 0.5. It is not possible for the quiz to result in a tie.

The ramblers won the quiz in 1996.
(i) Draw a probability tree for the three years t0 1999.
I drew tree correctly - no problem.

(ii) Find the probability that the strollers will win in 1999.
I added up all probs - eg 0.147+0.105+0,045+0.075=0.372 - that was correct answer.

(iii) If the strollers win in 1999, what is the probability that it will be their first win for at least three years?

I just selected the RRS option on tree above - 0.147. But this was incorrect. The correct answer apparently is 0.395. But how do you get that?

(iv) Assuming that the strollers win in 1999, find the smallest value of n such that the probability of the ramblers winning the quiz for n consecutive years after 1999 is less than 5%.

For this one I did this:
0.5^x < 0.05
log(0.05)/log(0.5) = 4.32., therefore n = 5. But that was incorrect, answer was 8. But how?

Angus

2. You are given that the strollers won in 1999 so you should not add the probability that will happen.

3. Thinking again, if the Strollers win, then the probability of losing next year is 0.5 followed by 0.7 the following 2 years. Therefore for next 3 years probability is 0.5*0.7*0.7? But that gives 0.245?