Bunuel wrote:
If the probability of rain on any given day is 50%, what is the probability that it will rain on at least three days in a row during a five-day period?
A. 3/32
B. 1/4
C. 9/32
D. 5/16
E. 1/2
Important not to miss ' in a row'..
At least three days means three cases...
1) 3 days in a row..
So take these three days as ONE R and other two as N
So NNR..
You can place R in 3C1=3 ways
2) 4 days.....4 days in a row OR 3 days in a row and 4th day of rain separated by a non rainy day
So take these three days as ONE R and non-rainy day as N and 4th day as r
So rNR........ rNR, rRN, RNr,rRN
4 ways
3) 5 days in a row
All 5 are one so one way
Total- 3+4+1=8
All possibilities=2*2*2*2*2=32
Probability=8/32=1/4
what is wrong with your way..
Firstly you are complicating a simple way..
We use complement when we went to ease our calculations as normal scenario will have more casesIf talking of complement..
At least three days in a row means MAX two days in a row..(IS THE COMPLEMENT OF AT LEAST THREE DAYS IN A ROW = 1 - P(MAX TWO DAYS)???) 1) two days..
When there are 4 days of rains....RRNRR
When there are three days....RNNRR, NRNRR, ....many ways
When there are 2 days....RRNNN, NRRNN..and so on
2) one day..
4days is not possible
3 days..RNRNR
2 days ...NNRNR, and so on
3) none of the day
NNNNN... One way
So as you see it is more complicated, stick to normal
Thanks for your respond. I appreciate the feedback.
I was confused because when reviewing probabilities, you are thought that when you have the words "AT LEAST" to automatically use the complement. As a result, that is the method I followed.
and NOT as you stated above..so it would be RRNNN,NRRNN, etc? Is my understanding of complements wrong?
In the past I solved a problem related to selecting cards out of a 52 card deck, that stated P (at least the third draw). The complement of this was used 1 - p(two or fewer draws).
Thanks again for helping clarify this.