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Probability+Combinatorics

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by RSK » Fri Jan 27, 2012 9:28 pm
Hi guys,

Can someone tell me how to solve this question:

A medical researcher must choose one of 14 patients to receive an experimental medicine called Progaine. The researcher must then choose one of the remaining 13 patients to receive another medicine, called Ropecia. Finally, the researcher administers a placebo to one of the remaining 12 patients. All choices are equally random. If Donald is one of the 14 patients, what is the probability that Donald receives either Progaine or Ropecia?

Thanks!
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by rijul007 » Fri Jan 27, 2012 10:44 pm
Lets say Donald was selected for progain
the number of choices for for othe two would be 13 and 12
Similarly if he was chosen for ropecia
the number of choices for other two medicines would be 13 and 12

totla number of possibilities = 12*13 + 12*13
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by shankar.ashwin » Sat Jan 28, 2012 12:24 am
Let A,B and C be the 3 medicines.

We need to find probability Donald receives either A or B.

P(A or B) = P(A) + P(B)

P(A) = Donald receives Progaine = 1/14

P(B) = Donald receives Ropecia = 13/14 * 1/13 = 1/14

Therefore, P(A or B) = 2/14 = 1/7

P(C) does not matter here.. (i.e Ropecia)
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by LalaB » Sat Jan 28, 2012 1:32 am
Donald is among these 2 guys that will get either P or R. so, 2/14 =1/7
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