Correct Answer - Option 3 : 20
Given:
A.P is 21 ,42, 63 ,84, .........
Concept:
The formula for finding the n-th term of an AP is:
an = a1 + (n - 1) d
Calculation:
a1 = 21 , a2 = 42 , a3 = 63 ,a4 = 84 ......
⇒ d = a2 - a1 = 42 - 21 = 21
Let 420 be then nth term using an = a1 + (n - 1) d
⇒ 21 + 21n - 21 = 420
⇒ 21n = 420
⇒ n = 20
∴ 20th term is 420