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Source: — Problem Solving |
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by shankar.ashwin » Sun Sep 25, 2011 6:42 am
Sum of n natural Nos is n(n+1)/2

Only 2 digit = (Sum of 1 to 99) -( Sum of 1-9)

(99)(100)/2 - (9) (10) /2 = 4905.

No answer choices though
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by knight247 » Sun Sep 25, 2011 6:46 am
For sets where all the elements are equally spaced

average=(biggest+smallest)/2=(10+99)/2=109/2

Also, for evenly spaced sets

Number of elements=[(Biggest-Smallest)/Increment]+1=(99-10)+1=89+1=90

Sum=Average*Number of elements=(109/2)*90= 4905 which doesn't seem to be in the answer choices at all
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by sparkle6 » Sun Sep 25, 2011 10:13 pm
Yes, I got 4905 as well. Must be an error in the problem. Thanks!
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