Answer:
Given,
First term, a = 5; last term, l = an = 80
Common difference, d = 3
an = a + (n – 1) d
an = 5 + (n – 1)3
= 5 + 3n – 3
= 3n + 2
Put an = 80 as 80 is last term of A.P.
3n + 2 = 80
3n = 80 – 2
3n = 78
n = 78/3
= 26
Total 26 terms exists in the A.P.