There are 7880 ball of radius 1 cm, if the ball are arranged in a form of pile such that top has 1 ball,2nd layer has 3,3rd has 6,4th has 10,5 has 10 and so on.how many levels are there.
Option
a. 33
b. 32
c. 34
d. 31

• The ques should be 1 ball at 1st layer, 3 at 2nd , 6 at 3rd , 10 at 4th, 15 at 5th ....... Then the series will become 1,3,6,10,15,21 ..... so on . Now taking first and second term together it will add up to 4 next two to 16, next two to 36 next two to 64 and so on. Den series will become 4,16,36,64, ... Take 4 common 4 (1+2*2+3*3+4*4 .....)
  So 7880=4(1+2*2+3*3+4*4 .....)
  7880*3/2=(N)(N+1)(2N+1)
• 10 years agoHelpfull: Yes(5) No(1)
• 34
  as it is in the form of n*(n+1)/2
  equation is n*(n+1)/2+n*(n+1)(2n+1)/6=7880
  it gives u n=34
• 10 years agoHelpfull: Yes(2) No(11)
• how did u gt n*(n+1)/2+n*(n+1)(2n+1)/6=7880 ???
• 10 years agoHelpfull: Yes(2) No(0)
• 1+3+6+10....n term
  sum=n(n+1)(n+2)/6
  hene n=34
• 10 years agoHelpfull: Yes(1) No(6)
• n(n+1)(n+2)/6=9880
  
  ans -38
• 7 years agoHelpfull: Yes(1) No(0)
• @Manu Please Explain More :/
• 9 years agoHelpfull: Yes(0) No(0)