Question

Fin the sum of odd number between 100&200

Answer 1

sum=0

for n=101 to 199 step 2 {
  sum += n
}

or, by hand, pair them up:
101 + 199 = 300
103 + 197 = 300
...
how many such pairs are there?

Or, since the sum of the 1st n odd numbers is n^2,

100^2 - 50^2 = 7500