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

In the given AP, the first term = a = 21

Common difference = d = 42 - 21 = 21

To find: place of the term 210.

So, let a_n = 210

Since, we know that

a_n = a + (n - 1) × d

∴ 210 = 21 + (n - 1) × (21)

⇒ 210 = 21 + 21n - 21

⇒ 210 = 21n

⇒ n = 10

∴ 10^th term of the AP is 210.