Answer:
18 book copies
Step-by-step explanation:
As you can see, there is a pattern where you add the next number after x. To find where to start, let's subtract the numbers we were given:
6 - 4 = 2
9 - 6 = 3
13 -9 = 4
So our next number is to add 5 to 13: 5 + 13 = 18, so in February the bookstore will order 18 books.
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