2475.
There are some easy ways to calculate this. Imagine adding up all the odd number from 11 to 99, and from 99 to 11, in two rows, so that 11 is paired with 99, 13 is paired with 97, and so on. Each pair sums to 110, and there are 45 pairs. So 45*110 is 4950, divided by two is 2475. Even easier if we factor the two out of the 110 first and simply multiply 45 by 55.
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The sum of an odd number of odd numbers will be odd.
5625
Any two odd numbers will have an odd product and an even sum.
An odd number of odd numbers can't add up to an even number.
The sum of the first 100 odd numbers is 10,000.