The given sequence is
[tex]\lbrace3,8,13,18,23,28,33,...\rbrace[/tex]
Since
8 - 3 = 5
13 - 8 = 5
23 - 18 = 5
Then it is an arithmetic sequence with common differences 5
We need to find
[tex]\sum_{n\mathop{=}4}^7a_n[/tex]
That means we need the sum of the terms from n = 4 to n = 7
The 4th term is 18
The 7th term is 33
Then the sum is
[tex]\begin{gathered} S=18+23+28+33 \\ \\ S=102 \end{gathered}[/tex]
The answer is c
