There are 49 students in a class.Kiran told that sum of all roll numbers below his number and after his number are equal.find the roll number of kiran

• the roll number of kiran is 35...
  suppose n+1 is kiran's roll number
  sum of roll's above kiran=n(n+1)/2 [sum of n natural numbers]
  sum of roll's below kiran=[49(50)/2]-[(n+1)(n+2)/2]
  [sum of numbers upto 49 - sum of numbers up to kiran's roll num(n+1)]
  =>n(n+1)/2=[49(50)/2)-[(n+1)(n+2)/2]
  =>n(n+1)+(n+1)(n+2)=49(50)[take n+1 common on lhs]
  =>(n+1)(2n+2)=49(50)
  =>(n+1)(n+1)=49(25) [dividing by 2 on both sides]
  =>n+1=7(5) [squre root]
  =>n=34
  
  so, kiran's roll number is n+1=35...
• 11 years agoHelpfull: Yes(8) No(4)
• total sum of roll number=49(49+1)/2=1225
  let kiran's roll num=x;
  student before her=x-1;
  student after her=49-x;
  total sum of ages of x-1 student=(x-1)(x-1+1)/2=x(x-1)/2;
  so,x(x-1)/2=(1225-x)/2
  =>x^2-x=1225-x
  =>x=sqrt(1225)
  =>x=35
  kiran's rol=35
• 11 years agoHelpfull: Yes(8) No(0)
• simply answer is 35...i too agree...but technique i found is
  step 1)by calculating sum of natural numbers(here 49) gives total value(1225)..
  step2)calculate square root of tat number gives you answer..so in general its to find sum and take square of tat number gives you exact or approxiamte answer
• 11 years agoHelpfull: Yes(4) No(1)
• dear Pragkarma ,it is given that sum of roll numbers below after are equal .your answer is wrong
• 11 years agoHelpfull: Yes(2) No(1)
• the roll number of kiran is 25,because above and below are 24(equal)
  25(ans)
• 11 years agoHelpfull: Yes(1) No(11)
• didnt get the question!
• 11 years agoHelpfull: Yes(0) No(0)