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

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Step 1: Numbers divisible by 5 but not by 2:
$5, 15, 25, 35, \ldots, 995$
Step 2: $a = 5$, $d = 10$, $a_n = 995 \Rightarrow a + (n-1)d = 995$
Step 3: $5 + (n-1)10 = 995 \Rightarrow (n-1)10 = 990$
$\Rightarrow n - 1 = 99 \Rightarrow n = 100$

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