Question 830780
You are wrong. The book is right.
The first day (which is not a week) 115 was deposited

after one week 35 is added
after two weeks another 35 is added
after 30 weeks how much is there?
read the question carefully
"30 weeks after his initial deposit"
one week after the first 35 is added
You are making it more complicated than it need be
t=35n+115