26588

I worked out 40+41+42+...+99+100 by first finding the sum of the missing piece (780). Then I subtracted the missing piece from the sum of 1 to 100 (4,270).

Charlies method can't work for sequences that don't go up in ones beacuse you will always end up with an odd number and you can't divide an odd number into equal parts. There is another way to do it. You make an exquation (n+1)x(n divided by 2) then you divide the total by 2. If you were doing 3c the answer would be 2,130.