Q. consider a series 1,2,3,....N, is divisible by 10, where N

• Sn=n(n+1)/2
  Therefore, n(n+1)/2 must be divisible by 10
  Or, n(n+1) must be divisible by 20
  
  CASE 1: take the following consecutive numbers
  4,5,6 [4*5 divisible by 20]
  14,15,16 [15*16 divisible by 20]
  24,25,26 [24*25 divisible by 20]
  34,35,36 [35*36 divisible by 20]
  44,45,46 [44*45 divisible by 20]
  .
  .
  .
  984,985,986 [984*985 divisible by 20]
  994,995,996 [995*996 divisible by 20]
  From above steps it is clear that there is
  an alternative number around the 5's multiple
  which is producing the combination to
  produce a number divisible by 20.
  COUNT_1=100
  
  CASE 2: take the following consecutive numbers
  19,20,21 [19*20 and 20*21 both are divisible by 20]
  39,40,41 [39*40 and 40*41 both are divisible by 20]
  59,60,61 [59*60 and 60*61 both are divisible by 20]
  79,80,81 [79*80 and 80*81 both are divisible by 20]
  .
  .
  .
  979,980,981 [979*980 and 980*981 both are divisible by 20]
  From above steps it is clear that both the numbers
  around the 20's multiple are producing the combination to produce a number divisible by 20.
  COUNT_2= 49*2=98
  
  CASE 3:
  The series 1 to 999 is divisible by 20
  COUNT_3=1
  
  Total Count = Count_1 + Count_2 +Count_3 = 100+98+1 = 199
  
• 11 years agoHelpfull: Yes(18) No(0)
• complete question is a series 1,2,3,4...n is divisible by 10 and n is less than 1000.for example 1,2,3,4. sum is divisible by 10,how many search series are possible?
• 11 years agoHelpfull: Yes(6) No(1)
• Sn=n(n+1)/2 ;n
• 11 years agoHelpfull: Yes(5) No(3)
• Today's Ques.
  consider a series 1,2,3,....N, is divisible by 10
  where N
• 11 years agoHelpfull: Yes(0) No(1)
• consider a series 1,2,3,....N, is divisible by 10 where N
• 11 years agoHelpfull: Yes(0) No(1)
• this question is only solved by option.
  
• 11 years agoHelpfull: Yes(0) No(2)
• options are
  1-> 199
  2-> 249
  3-> 399
  
• 11 years agoHelpfull: Yes(0) No(1)
• Sn=n(n+1)/2 ;n
• 11 years agoHelpfull: Yes(0) No(2)
• Sn=n(n+1)/2 ;n
• 11 years agoHelpfull: Yes(0) No(2)
• 1,2,3.......15
  sum:n(n+1)/2 must be divisble by s10
  15*16/2=60 is divisble by 10
  answers are n=15,20,24,...
• 11 years agoHelpfull: Yes(0) No(1)
• its answer is 199, 249-50 = 199;
• 11 years agoHelpfull: Yes(0) No(1)
• sum of n terms of an series 1,2,3,4...........n is
  n^2+n/2 for divisible by 10 it must produce value of type 10*k
  hit and trail
  n=4,15,20 and so on...
• 11 years agoHelpfull: Yes(0) No(1)
• Sn=n(n+1)/2 ;n
• 11 years agoHelpfull: Yes(0) No(0)
• Ans:-199
  Approach:-
  Sn=n(n+1)/2 for n
• 11 years agoHelpfull: Yes(0) No(0)