A probability question - could anyone help?

[monokekie]

Hey, it goes like this:

When doing blood testing for HIV infections, the procedure can be made more efficient and less expensive by combining samples of blood specimens, If samples from three ppl r combined n the mixture tests negative, we know that all three individual samples are negative. Find the probability of a positive result for three samples combined into one mixture, assuming the probability of an individual blood sample texting positive is 0.1.

the answer is really small which i don't really get..

anyhelp will be very appreciated!

[stonecold]

okay, i'm no expert.  but maybe it would be Pr(1 person is +) + Pr(2 people are +) + Pr(3 people are +)

so 0.1 + 0.1x0.1 + 0.1x0.1x0.1

Pr = 0.111

[stonecold]

haha, nah i'm probably wrong.  but 1/1024 sounds too small.  the Pr of one person testing positive is 1/10, so the Pr would increase if you tested more people lol...

[kamil9876]

1/1024 sounds like an answer to a different question!. 1024=2^10 so it might come from tossing coins 10 times or other problems where each trial has two outcomes equally probable.

[stonecold]

^ i just realised that then kamil....aaargh!

been a while since i've done prob haha.  i forgot you've gotta include the complement!