. The integers 1, 2, …, 40 are written on a blackboard. The following operation is then repeated 39 times: In each repetition, any two numbers, say a 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 board at the end?

• Qn is not clear.
  
  If statement is
  
  The following operation is then repeated 39 times: In each repetition, any two numbers, say a and b, currently on the blackboard are erased and a new number
  ( a + b -1) is written, then
  
  number left on board will be
  (1+2+3+...40)-39= (40*41/2 )-39 = 820-39 = 781
• 12 years agoHelpfull: Yes(3) No(0)
• Step-1
  1, 40 are erased => 1+40-1 = 40 is the new number written
  2, 39 are erased => 2+39-1 = 40 is the new number written
  This pattern is repeated until
  20, 21 are erased => 20+21-1 = 40 is the new number written
  => we are left with twenty 40’s on board
  Step-2
  Two 40’s erased and 40+40-1 = 79 is the new number written, this pattern is repeated
  => we are left with ten 79’s on board
  Step-3
  Two 79’s erased and 79+79-1 = 157 is the new number written, this pattern is repeated
  => we are left with five 157’s on board
  Step-4
  Two 157’s erased and 157+157-1 = 313 is the new number written, this pattern is repeated again
  => we are left with two 313’s and one 157 on board
  Step-5
  {(313+313-1)+157}-1 = 781 the last number left on the board
  
  Ans is 781
• 8 years agoHelpfull: Yes(1) No(0)