The Integer 1,2,.........40 are written on 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?????
a. 820
b. 821
c. 781
d. 819
e. 780
7 comments:
781..
consider 4 numbers 1,2,3,4..now if we carry out the same operation on these 4:
1,2,3,4 = (1+2)-1,3,4 = (1+2+3)-2,4 = (1+2+3+4)-3
following this for the first 40 natural nos. we get
{1+2+3+.....+40} - 39 =
40*41/2 - 39 = 781
Aren't we supposed to take 2 numbers at a time & then subtract 1Akhil? Are u sure that this is the way this problem has to be interpreted?
yeah gd akhil....ans is 781...
aayushi actually we r erasing 2 nos...nd then writing them again by subtracting 1.
so, ds is nthng bt (1+2+3.....+40)-39...
nw try it..
ok..got it
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