Consider the sum of first 70 natural numbers. If every digit '6' is replaced by '8' what would be the net increase and the new sum?

• 2485+(12+180+22)
  net increase =214 ans.
  
  as 66 changes to 88 i.e. increased by 22
• 10 years agoHelpfull: Yes(5) No(0)
• sum n natural nos=n(n+1)/2
  for 70 numbers=70(71)/2=2485
  if digit 6 is replaced by 8 (6,16,26,36,46,56,60,61,62,63,64,65,66,67,68,69)
  becomes (8,18,28,38,48,58,80,81,82,83,84,85,86,87,88,89) new sum=2485+(12+200)
  net increase is 212
• 11 years agoHelpfull: Yes(3) No(3)
• Sum of 1st 70 natural no's=70*71/2=2485..till 70 there are 7 digits with 6 in units place...so netincrease is 7*2=14..from 60-69 except 66 there are 10 6's in 10's digit ..so the value is increased by 20*100=200.total increase is 200+14=214.
• 7 years agoHelpfull: Yes(2) No(0)
• new sum is 2485+214=2699
• 9 years agoHelpfull: Yes(0) No(1)
• n(n+1)/2=70*71/2=2485
  upto 70 there are 7 6s in units place(6,16,26,36,46,56,66)=7*2=14
  from 60 to 69 there are 10 6s in tens place=2*10(tens place)*10(ten times)=200
  then =200+14=214(net increase)
  total=2485+214=2699
  
• 9 years agoHelpfull: Yes(0) No(1)
• (70 *71)/2=2485
  (70*71)/2 +257=2742
  2742-2485=257
  (8,18,28,38,48,58,80,81,82,83,84,85,86,87,88,89)=257
• 9 years agoHelpfull: Yes(0) No(1)
• Sum of 1st 70 natural no's=70*71/2=2485..till 70 there are 7 digits with 6 in units place...so netincrease is 7*2=14..from 60-69 except 66 there are 10 6's in 10's digit ..so the value is increased by 20*100=200.total increase is 200+14=214.
• 7 years agoHelpfull: Yes(0) No(1)