Question

Which term of the AP, $21,42,63,84,...$ is $210?$

• A. $9^{th}$
• B. ${10}^{th}$
• C. ${11}^{th}$
• D. ${12}^{th}$

Answer 1

The correct option is B ${10}^{th}$

First term of the AP, $a=21$
Common difference $d=21$
$n^{th}$ term is $210$
Then,
$210=a+(n-1)d$
$210=21+(n-1)\times 21$
$210=21n$
Therefore, $n=10$
Hence, 210 is ${10}^{th}$ term of the series.