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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Sequences
Source: Beat The GMAT — Problem Solving |
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
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
