Answer:
Andrew has been in the book club for 18 months.
Explanation:
The number of books already read by Andrew = 13 books
He reads 1 book each month after joining the book club.
Number of books he has read after joining the club = 31 books
Number of books read after joining the Club
= Total books Read - Books read beforehand
= 31 books - 13 books
= 18 books
So, the number of books Andrew read within the club = 18 books
Now, as Andrew read 1 book per month
⇒ Andrew reads 18 books in 18 x ( 1 Month) = 18 Months
Hence, Andrew has been in the book club for 18 months.