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

(b) 10th

Here, a = 21 and d = (42 - 21) = 21
Let 210 be the nth term of the given AP.
Then ​Tn = 210 
⇒​ a + (n - 1)d = 210
⇒​ 21 + (n - 1) ⨯ 21= 210
⇒ 21n = 210
⇒ n = 10
Hence, 210 is the 10th term of the AP.