Previous Year Paper
CAT2008Previous Years Paper
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 erased and a new number a + b – 1 is written. What will be the number left on the board at the end?
A  820

B  821

C  781

D  819

E  780

Let S = 1 + 2 + ... + 40. So 1 + 2 + a + b +... + 40 = S
After 1st step:
1 + 2 + .... + (a + b — 1) + ... + 40 = S — 1
So, after each step the total reduces by 1. Continuing this way after 39 steps numbers left S — 39 = – 39 = 781
Hence, (c)
CAT2008Previous Years Paper Flashcard List
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