gmatprep - sum of sequence

shahab03 wrote:why isnt a-b = 29

since sum of a terms = 110
and sum of b terms = 81

plz explain.. thanks

I am getting 10 as the answer. I cant see how 20 could be the OA.

a = sum of even integers from 2-20 inclusive, 10 integers
a = 10/2 [ 2*2 + (10-1)*2] = 5 [4+18] = 5*22 = 110

b = sum of odd integers from 1-19 inclusive, 10 integers
b= 10/2 [ 2*1 + (10-1)*2] = 5[2+18] = 5*20 = 100

a-b = 110 - 100 = 10

if 20 is the OA, then its wrong, according to me.

a+b = sum of all integers from 1-20 inclusive.

a+b = 20/2 [1*2 + (20-1)*1] = 10[2+19] = 10*21 = 210

Therefore a-b has to be 10 and not 20.

Let me know guys if I am missing something.

OA is B. thx

a-b = 10

has to be 10...i think u missed out on 19 in b

thats why ur sum total for b is coming to 81 rather than 100