Question

which term of AP 21,42,63,84,...is 210

Answer 1

Nth term of ap= a+(n-1)d
here a= 21.....; d = 42-21=21......;Nth term= 210
so
210=21+(n-1)21= 21+21n-21
210= 21n
n=210/21=10
so 10th term is the required answer

Answer 2

An = 210 A = 21 d = 21

An = A+( n-1)d

210 = 21+(n-1)21
189 = (n-1)21

$\frac{189}{21} = n - 1$
9 = n-1
n = 10