How many natural numbers are there between 1 and 1000 . . .

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

Doubt by CBSE

Solution : 

Natural numbers which lies between 1 and 1000 which are divisible by 5 but not by 2 are :
5, 15, 25, . . . 995

Here
a = 5

d = a₂-a₁

d = 15-5
d = 10

an=995

a_n=a+(n-1)d
995=5+(n-1)(10)

995-5=(n-1)10

990/10=n-1
99+1=n

100=n

n=100

Hence, there are 100 natural numbers between 1 and 1000 which are divisible by 5 but not by 2.