How to solve this one... can someone pls give detailed steps?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?

Hi Prasanth
The question is all about observing a pattern. Say you remove 5 and 4, so you what will be adding is 8. So effectively each time you are performing the operation, the overall sum of numbers is getting reduced by 1. In 39 operations, the sum will get reduced by 39. Initial sum is 40*41/2 = 820
So new sum would be 820 - 39 =781. Thus the last number remaining would be 781.