Question:
Choose the correct answer in each of the following questions:
How many two-digit numbers are divisible by 3?
(a) 25
(b) 30
(c) 32
(d) 36
Solution:
The two-digit numbers divisible by 3 are 12, 15, 18, ..., 99.
Clearly, these number are in AP.
Here, a = 12 and d = 15 − 12 = 3
Let this AP contains n terms. Then,
$a_{n}=99$
$\Rightarrow 12+(n-1) \times 3=99 \quad\left[a_{n}=a+(n-1) d\right]$
$\Rightarrow 3 n+9=99$
$\Rightarrow 3 n=99-9=90$
$\Rightarrow n=30$
Thus, there are 30 two-digit numbers divisible by 3.
Hence, the correct answer is option is B.