Question

Hwo many numbers are there between 101 and 999, which are divisible by both 2 and 5?

Answer 1

a = 110 {first number to be divided by 2 and 5}
difference d =10
an=990 {last no to be divided by 2 and 5}
$a_n=a+(n-1)d$
990 = 110 + 10n -10
880 + 10 = 10n
890 = 10n
n = 89
∴ number of natural numbers between 101 and 999 is 89.