SOLUTION: how many odd numbers are there between 100 to 1000?

That's the sequence 101, 103, 105, ... 997, 999

That's an arithmetic sequence with a1 = 101
an = 999, and d=2 

an = a1 + (n-1)d

999 = 101 + (n-1)2

898 = 2(n-1)
898 = 2n-2
900 = 2n
450 = n

Edwin