Probability, very very simple question
There’s no mark scheme so I just wanna quickly check . Is the answer to this question 0.74 ?
(edited 4 years ago)
Original post by Leah.J
There’s no mark scheme so I just wanna quickly check . Is the answer to this question 0.74 ?
Hi Leah, I don't know if it's just my computer but I can't read the question properly. There may be a few words missing. What comes between "The probability that this student is from schoo" and "doesn't have a laptop equals" ?
No, it's not 0.74. There are 100 students. If you asked each student "Say YES if you are from school A or if you don't have a laptop" how many of them would say YES ?
Original post by MarkFromWales
No, it's not 0.74. There are 100 students. If you asked each student "Say YES if you are from school A or if you don't have a laptop" how many of them would say YES ?
Oh I counted 15 ppl twice , so 0.59 ?
Original post by Leah.J
Oh I counted 15 ppl twice , so 0.59 ?
Yes, well done.
Original post by Thehorsegal
yes pretty sure you are correct as it is the union of the two events so you just add the probability of going to school a (0.52) to the probablilty of not having a laptop (0.22) whis is indeed 0.74 
No, that doesn't work because the two events aren't mutually exclusive.
It's 0.59 because suppose there are two events A and B, the probability of their union is P(AUB) = P(A) + P(B) - P(AnB). This is a standard formula of probability theory. Suppose in this case that P(A) is the probability of coming from School A and P(B) is the probability of not having a laptop. So their union is equal to 0.52 + 0.22 - 0.15 = 0.59
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