Find the sum of the arithmetic progression

Question

4 + 9 + 14 + 19 + …….94 + 99 + 104

Answer 1

first you have to know how many terms you have
what term is 104
a + (n-1)d = term(n)
4 + (n-1)(5) = 104
5n - 5 = 100
5n = 105
n = 21

now sum(21) = (first + last)(21/2)

=(4+104)(21/2) = 1134