Question 813533
<pre>That's the sequence 101, 103, 105, ... 997, 999

That's an arithmetic sequence with a<sub>1</sub> = 101
a<sub>n</sub> = 999, and d=2 

a<sub>n</sub> = a<sub>1</sub> + (n-1)d

999 = 101 + (n-1)2

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

Edwin</pre>