help math

The smallest odd integer is $301$, and the largest one is $599$. There are $150$numbers in this series, so the sum is $150 \times 450 = \color{brown}\boxed{67,500}$

Between 300 and 600 - 150 odd integers.

$\displaystyle\sum_{n=1}^{150} \mbox{ } ( 2n + 299 ) = 67500$, as BB said.