This can be found 2 ways: a) the long way 1+3+5+7+9+11+13+15+17+19= 100 or b) the twentieth odd number is 19. 1 +19 = 20, 3+ 17 = 20, etc. there are 5 pairs of this meaning five pairs of numbers that add up to 20. 5x20 = 100.
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1,3,5,7,9,11,13
The sum of the first 100 odd numbers (1 through 199) is 10000 (ten thou)
The sum of the first 300 odd numbers is 90,000
The sum of the first 40 odd numbers is 1,600.