Im taking up advanced functions and relations and this questions (for some reason) has me stumped. I think i got the right answer but the answer in the back of my book is different:

Suppose you earn 25 cents on September 1, 50 cents on September 2, 75 cents on September 3, and so on. How much money will you earn by the end of the month?

The answer in the back is 116.25$.

I got 120.

Btw this has to deal with a general formula:

t(n)= a + (n-1)d where A is the first term and D is the difference between each term.