Question:
Choose the correct answer in each of the following questions:
How many three-digit numbers are divisible by 9?
(a) 86
(b) 90
(c) 96
(d) 100
Solution:
The three-digit numbers divisible by 9 are 108, 117, 126, ..., 999.
Clearly, these numbers are in AP.
Here, a = 108 and d = 117 − 108 = 9
Let this AP contains n terms. Then,
$a_{n}=999$
$\Rightarrow 108+(n-1) \times 9=999 \quad\left[a_{n}=a+(n-1) d\right]$
$\Rightarrow 9 n+99=999$
$\Rightarrow 9 n=999-99=900$
$\Rightarrow n=100$
Thus, there are 100 three-digit numbers divisible by 9.
Hence, the correct answer is option D.