Answer
Formula for $a_{n}$ term of the given sequence = $a_{1}$ + (n - 1) d.
Where
$a_{1}$ = first term
d = Common difference
n = $n^{th}$ number of term
$20^{th}$ term of sequence = 77
Work Step by Step
Given arithmetic sequence = 1, 5, 9, 13- - - - -
Common difference between consecutive term = 13 - 9 = 9 - 5 = 5 - 1 = 4.
Formula for $a_{n}$ term of the given sequence = $a_{1}$ + (n - 1) d.
Where
$a_{1}$ = first term
d = Common difference
n = $n^{th}$ number of term
$20^{th}$ term of sequence = 1 + (20 - 1) $\times$ 4
= 1 + 19$\times$4
= 1 + 76
= 77
