Solve :

Which term of the arithmetic progression 84, 80, 76 … is zero ?

Let the n^th term of the progression be 0.
First term a = 84 and common difference = 80 – 84 = -4
Since, a_n = a + (n -1)d

Where,

a = First term of AP
d = Common difference of AP
and no of terms is ‘n’

∴ 0 = 84 + (n – 1) × (-4)
⇒ 0 = 84 -4n -4(-1)
⇒ 0 = 84 -4n + 4
⇒ -88 = -4n
⇒ n = 22

Hence, Zero is the 22^nd term of the progression.