Latest Others Aptitude Question SOLUTION: . 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 blac

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(#M40012581) OTHERS question Keep an EYE

. 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?

Asked In Others manu (11 years ago)

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pawan  (11 years ago)

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

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KUMAR HARSH  (8 years ago)

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

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