Question

quantos termos tem a p.a. (15,20,25,...,160)?

Answer 1

an = a1 + (n -1 ) . r
160 = 15 + ( n - 1 ) . 5
160 = 15 + 5n - 5 
160 = 10 + 5n
160 - 10 = 5n
5n = 150 
n = 150/5
n = 30      ok

Answer 2

an= a1 + (n-1)r
160= 15 + (n-1)5
160=15 + 5n -5
160= 10 + 5n
5n=160-10
5n=150
n=150/5
n=30 termos