the number 1,2,3......40 are written on blackboard. the following operation is then repeated 39 times .in each repetition any two number say and b currently on the blackboard are erased and a new number a+b-1 is written what will be the number left on the blackboard at the end.
options
a) 820 b)821 c)781 d)819 e)780

• c) 781
  total sum = 1+2+3+....+40 = 40*41/2 = 820
  (a+b) is erased and (a+b-1) is written , so net effect is (-1) ,no matter values of a,b
  for 39 times = -39
  at the end = 820-39 = 781
• 9 years agoHelpfull: Yes(3) No(0)
• no of pairs=20
  20(n-1)=20*39=780
• 9 years agoHelpfull: Yes(0) No(1)