Latest Infosys Aptitude Question SOLUTION: 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

• Placement Papers
• /
• Infosys
• /
• Numerical Ability
• /
• Probability

Infosys Question Papers: Infosys Interview Experience(10) INFOSYS Company Profile

(#M40032515) INFOSYS Numerical Ability question Probability Keep an EYE

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?

Asked In Infosys Satyaranjan (10 years ago)

Solved sameer Read Solution (6)

Is this Puzzle helpful?   (49)   (17) Submit Your Solution Probability

Read Solution (6) :

sameer  (10 years ago)

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

Is this solution Helpfull? Yes (45) | No (20)  

nita  (10 years ago)

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

Is this solution Helpfull? Yes (25) | No (9)  

mithun bhaumik  (10 years ago)

(60/1000 * 1/800)+(60/800 * 1/1000) =3/20000

Is this solution Helpfull? Yes (15) | No (1)  

archit bohra  (5 years ago)

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.

Is this solution Helpfull? Yes (5) | No (4)  

Ishan Shrivastava  (8 years ago)

Probability = 60C1 / 1000C1 x 800C1
Ans = 3/40000

Is this solution Helpfull? Yes (2) | No   

lakshya sharma  (5 years ago)

60c1/(1000c1*800c1)=60/800000=3/400000

Is this solution Helpfull? Yes | No (1)  

Please Login to Read Solution.