Why is my answer for the sum of arithmetic series off by 53 each time

Ok, so we're doing arithmetic series in math and the equation we got is Sn=n/2(t1+tn)
But when I do that, no matter the question, my answer is always wrong by 53.
For example: The question is Detrmine the sum of each arithmetic series.
a)5+8+11+...+53 I figured out that n is 16.
So Sn=16/2(5+15)
I get 440, but the answer in the back of the book is 493.

Same when I do the next one:
b)7+14+21+....+98 n=13
Sn=13/2(7+98)
I get 682.5, but the answer is 735.

I get the answer wrong by 53 when I do the next one and the next one and the next one. What am I doing wrong? Also, I've tried the equation Sn=n/2(2t1+(n-1)d) and I still get the wrong.
What equation do I use? What's the difference between the equations? And why am I wrong by 53 each time?

It looks like you are counting the terms wrong. In a), there are 17 terms. The first is 5, the last is 5 + 16*3 = 5+48=53. 16 is the number of increases, not the number of terms--there is not increase reflected in the first term.

Sn = 17/2(5+53) = 493
not 16/2(5+53) = 464
you wrote 16/2(5+15) which is 160. How did you get 440? That's 5x55, but I can't see how you got 55.

b) 14/2(7+98) = 735
not 13/2(7+98) = 682.5. You should know that is wrong just by looking at it since the sum of any bunch of integers can't be a mixed number, 682 1/2.