There are 1000 junior and 800 senior students in a class.And there are 60 sibling pairs where each pair has 1 junior and 1 senior.1 student is chosen from senior and 1 from junior randomly.What is the probability that the two selected students are from a sibling pair?

• junior student=1000
  senior student=800
  60 sibling pair=2*60=120 student
  1 student choosen frm senior=800c1=800
  1 student choosen from junior=1000c1=1000
  therefore,1 student choosen frm senior and 1 student choosen from junior n(s)=800*1000=800000
  two selected student are from a sibling pair n(E)=120c2=7140
  therefore,P(E)=n(E)/n)(S)=7140/800000=714/80000
  
  
• 10 years agoHelpfull: Yes(45) No(20)
• The Probability of selecting 1 sibling junior = 60/1000 and the senior who is his sibling is 1/800 ..sso probability of seeelecting sibling pair is (60/1000)*(1/800) ,so ans is 3/40000
• 10 years agoHelpfull: Yes(25) No(9)
• (60/1000 * 1/800)+(60/800 * 1/1000) =3/20000
• 10 years agoHelpfull: Yes(15) No(1)
• All the given answers are wrong. Here's how.
  
  Mithun: Your answer is fundamentally incorrect. You are including duplicate cases. Please solve again properly. You'll surely find out the error. (Both your terms are same and redundant)
  
  Nita, Ishan, Lakshaya: The flaw is in the logic: You assumed that the two people have to be the SIBLING PAIRS, while the question says, that you just need 2 people who are siblings(but not of each other!!, of anyone!). Now, you might argue saying that the question reads "a sibling pair". But then it is not a problem with us. It is a problem with the question. I'd have loved if you mentioned this ambiguity in your answer. Nevertheless, let's move ahead.
  
  Now, Sameer did a great job understanding the Sibling part, but did a mistake in the 120C2 part.
  
  The moment you choose 2 from 120, you are violating the rule that you need to pick 2 students from each category: Senior and Junior. While, your way includes the case when 2 SENIOR Students OR 2 Junior Students are selected. So, it's wrong.
  
  The correct way is:
  Ways to pick one Sibling Senior: 60
  Ways to pick one Sibling Junior: 60
  Ways to make their combinations: 60*60.
  Similarly, total ways of selecting one junior, one senior: 1000*800
  Therefore, required probability: (60*60)/(1000*800) = 9/2000.
• 6 years agoHelpfull: Yes(5) No(4)
• Probability = 60C1 / 1000C1 x 800C1
  Ans = 3/40000
• 9 years agoHelpfull: Yes(2) No(0)
• 60c1/(1000c1*800c1)=60/800000=3/400000
• 6 years agoHelpfull: Yes(0) No(1)