How many multiples of 9 are there between 100 and 2,000?

First of all, find the first number more than 100 that can divided by 9, and it's 108 (we'll call it as 'a') then, find the number less than 2000 that can divided by 9, it's 1.998 (we'll call it as 'Un')

Un = a + (n-1). b

1998 = 108 + (n-1).9

1998 = 108 + 9n - 9

1899 = 9n

211 = n

and the total is

Sn = n/2 (a+un)

s211 = 211/2 (108+1998)

= 222.183