how many books has she read so far? She's been doing this for 7 months, so so far she's read:
7*3=21
how many does she still need to read?
30-21=9
so she needs to read 9 more books. With 3 books a month, this means:
9/3=3
so she needs to read 9 more books, which will take her 3 months!