If you can add these all in your head, then bravo to you

Sum of n terms of an Arithmetic Progression where a and l are first and last terms respectively is -

$\frac{n}{2}\times(a + l)$

Given that the sum of the Arithmetic Progression 2,4,6,...,999998 is W,

$W=\frac{n}{2}\times(2 + 999998)$

$\implies n=\frac{2W}{1,000,000}$

Now, there will be 'n+1' terms in the AP of odd numbers less than 1,000,000. Let the sum of odd numbers less than 1,000,000 be S.

$S=\frac{n+1}{2}\times(1 + 999999)$

Substituting value of n in the above equation,

$S=(\frac{W}{1000000}+\frac{1}{2})\times(1000000)$

$\therefore S = W+500,000$