How many natural numbers are there between $\displaystyle 1$ and $\displaystyle 1000$ which are divisible by $\displaystyle 5$ but not by $\displaystyle 2$ ?

Numbers divisible by $\displaystyle 5$ but not by $\displaystyle 2$ : \[\begin{array}{l} 5,15,25,35, \ldots, 995 \\ a=5, d=10, a_{n}=995 \Rightarrow a+(n-1) d=995 \\ 5+(n-1) 10=995 \Rightarrow(n-1) 10=990 \\ \Rightarrow n-1=99 \Rightarrow n=100 \end{array} \]