15 students join a summer camp. every day 3 students are on duty after the school to clean classrooms. After the course it was found that every pair has been on duty exactly once. How many days does the course last for ?

• 15c3= 15*14*13/3*2*1=455
  there can be different pairs of 3 among 15 students so 15c3
• 12 years agoHelpfull: Yes(22) No(13)
• each student can make a pair with every other student in 15c2 ways, i.e. ((14*15)/2)=105. again, let no. of days be x.
  th4, total no. of students working there = (3*x)
  => 3*x=105 => x=35 (ans)
• 12 years agoHelpfull: Yes(20) No(7)
• 5 days.. there r 15 students n 3 student pair do the work only once then 15/3 = 5 days..
• 12 years agoHelpfull: Yes(13) No(25)
• here is it necessary to take combination i didn't think so ans 5days
• 12 years agoHelpfull: Yes(5) No(9)
• total no. of pairs=15c2=105
  total no. of pair working each day=105/3=35
  no. of days camp lasted=105/35=3days
• 12 years agoHelpfull: Yes(4) No(14)
• answer should be 105days..
• 12 years agoHelpfull: Yes(4) No(5)
• 15/3..
  =5 days
• 12 years agoHelpfull: Yes(3) No(8)
• 15c3
  as 3 students are to be selected from 15.
• 12 years agoHelpfull: Yes(3) No(3)
• it is saying that out of 15student each possible pair that can be form out of 15 has done cleaning exactly once ..... so number of possible pairs that can be form out of 15 is 15c2=105. so total 105 pairs and in question 1 pair exactly once so total 105 days couse last
  
• 12 years agoHelpfull: Yes(3) No(1)
• y so can u explain it
• 12 years agoHelpfull: Yes(2) No(4)
• let us say there r 15 students then
  A B ( 13 way ) = 13 paired AB with other student
  C D ( 11 way ) = 11 now paired CD with other remaining student
  E F ( 9 way ) = 9 paired with EF
  G H ( 7 way ) = 7 paired with GH
  and so on
  therefore 13+11+9+7+.....1 = 49 is the answer.. therefore 49 days the course last.
  
• 12 years agoHelpfull: Yes(1) No(3)
• ans shud b 15c3=455
• 12 years agoHelpfull: Yes(1) No(2)
• 15C3 IS THE CORRECT ANSWER.........
• 12 years agoHelpfull: Yes(1) No(0)
• the options for these ques is a)35, b)45 c)105 and d)none of these .. so the correct answer for the que is ?????? . i got the same ques in the link-- ( http://learnwebdesigns.blogspot.in/2012/10/tcs-written-exam-guwahati.html )if anyone can bring the solution from the options mentioned are most welcomed.
• 12 years agoHelpfull: Yes(0) No(8)