For the life of me, I can't figure out how to do this probability problem. I have tried it over and over, but I know I am missing something. Because everytime my answer is not the same as the correct answer. What am I not doing?
A student has a box containing 25 computer disks, of which 15 are blank and the other 10 are not. If she randomly selects disks one by one, what is the probability that at least two must be selected in order to find one that is blank? (I know the answer is .4, I just don't know how to get it)
This is what I have done so far:
A=15, B=10
P(A)= 15/25 P(B|A)= 14/24
P(A)= 3/5 P(B|A)= 7/12
P(A)= .6 P(B|A)= .58
P(A)*P(B/A) = P(.6)*P(.58) = .35 = not right!!!
I have also tried:
P(B)*P(A|B) = P(.4)*P(.625) = .25 = not right!!!
I know I am not on the right path, obviously it isn't as straight forward as I think its supposed to be.
A student has a box containing 25 computer disks, of which 15 are blank and the other 10 are not. If she randomly selects disks one by one, what is the probability that at least two must be selected in order to find one that is blank? (I know the answer is .4, I just don't know how to get it)
This is what I have done so far:
A=15, B=10
P(A)= 15/25 P(B|A)= 14/24
P(A)= 3/5 P(B|A)= 7/12
P(A)= .6 P(B|A)= .58
P(A)*P(B/A) = P(.6)*P(.58) = .35 = not right!!!
I have also tried:
P(B)*P(A|B) = P(.4)*P(.625) = .25 = not right!!!
I know I am not on the right path, obviously it isn't as straight forward as I think its supposed to be.