Let b(t) be the number of unread books remaining on alex's bookshelf after reading consistently for t months.

b ( t ) = 75 - 5t

so, b ( 9 ) = .
options : 70 or 30 or 45 or 15

this means that ,
options :
* after 9 days
* after reading 9 books per month
* after reading 9 books
* after 9 months

the .
options :
* number of months that have passed is 70
* number of unread books remaining is 30
* number of unread books remaining is 45
* number of months that have passed is 15